I think we have strong reasons to believe that Tyrion is betraying Dany by the end.
Unfortunately, I feel that this is a fairly strong possibility. First of all we still have (in the books) the “betrayal for love” prophecy hanging over Dany. I don’t believe Jon will be the betrayer there. Tyrion is a more shady character than he has been portrayed in the show. The ending of the series and show are going to be the same, so if Tyrion is going that way in one it will happen in the other.
Now let’s look at the 7th season. Tyrion has constantly tried to restrain Daenerys. As he outright admits to Cersei, he has been trying to keep their family safe. This is the exact inverse of the impression he has given Dany (and one which she is very right to doubt). The one occasion on which Daenerys actually uses her forces is something he is extremely unhappy with. He is afraid for Jaime and afraid for what Daenerys’ military superpower status actually means for the Lannisters. For me one of the the most suspicious scenes in the whole season is the scene in Dany’s council room where he proposes the Wight Hunt plan. Remember that at this point in time, Dany does not really believe in the Northern threat (or does not believe in it as a serious threat). This is confirmed by her many times afterwards. She is still focused on Cersei. Even Cersei knows that it would be absurd to expect Dany to offer peace at this juncture because she just won a major victory and is winning the war. Tyrion knows this quite well. He tells Jaime that Daenerys will win this war. But Dany does have very strong growing feelings for Jon. Tyrion realizes this. The only way to possibly stop Daenerys from using her military might to annihilate Cersei and to offer a truce seems to be to manipulate these feelings to his advantage (notice that right before this scene in the council, Varys and Tyrion are discussing what happened to the Tarlys, and that they have to figure out some way to make her listen to them). So he does so. He argues that Cersei believes that the army of the dead is just a myth. So Jon should go an fetch a wight to give her evidence of the contrary. The problem with this is that his own Queen shares Cersei’s viewpoint on this question at this juncture. However, Daenerys wants to give Jon a chance to prove the claim he has been making ever since he showed up on her doorstep. Even Jon knows this (he tells Tormund that he needs to convince both Queens).
Tyrion sees that
1) Jon wants to go back to the North and face some dangerous threat in a war; and
2) That Daenerys does not want him going back into such a situation with inadequate military support.
He decides to leverage this to get a truce with the Lannisters. He argues that Jon be given a chance to prove this to Cersei (suppressing that Daenerys doesn’t believe it either at this point–something that she doesn’t want to say outright either, however). Daenerys is initially ready to sign off on this. She probably thinks it will keep Jon on Dragonstone (which is clearly where she wants him) and perhaps settle this issue once and for all (he hasn’t volunteered to go himself at this juncture, but Tyrion probably knew that he would, knowing who he was). But once she effectively signs off on it (a militarily bizarre course for her Hand to suggest because it gives her enemies respite for a non-existent threat), it is difficult to reverse course when Jon also volunteers. And so the stupid Wight Hunt plan initiates (even Jon knows its stupid). Dany declares s truce and agrees to meeting Cersei instead of attacking.
Then we have what happens in 7x07. We never see the end of the conversation between Tyrion and Cersei. Why would she agree? Tyrion knows her well. It is extremly doubtful that she would agree for some false reason which he wouldn’t see through at once. Cersei has never managed to pull the wool over Tyrion’s eyes so well. And yet he never suggests afterwards that she may be tricking Daenerys. Why not? Since when did Tyrion become so unsuspecting of her? That may be Jaime, but not Tyrion. More telling, however is Cersei telling Jaime that he was “the stupidest Lannister” for not realizing that she was not going to keep her word. Which suggests quite strongly that Tyrion realizes this already (otherwise why wouldn’t he be the stupidest?)
So I suspect that the traitor Tyrion theory has much merit. The show has gone out of its way to show Daenerys doing some things that would make Tyrion question his allegiance to her.
It would be sad, of course. Tyrion finally found someone who didn’t judge him for his stature and treated him with the respect he never found elsewhere. She clearly cares for him (she actually does care quite deeply for anyone in her inner circle–as the books make clear), hell even apologizes for making a joke that may seem unintentionally offensive to dwarves. And she has seen betrayal quite often.
I think the situation is far more complex than that, even though the show has not emphasized it in all cases. But they have indicated that. Daenerys’ no. 1 problem for now (before knowing about the Northern threat) is that Westerosi society is not willing to accept her as a Westerosi. This despite the fact that she has spent her whole lifetime thinking of Westeros as home and was born on Dragonstone. On this Cersei’s propaganda has been excellent.
Dany currently has an army of Dothraki, whom all of Westeros (and the world) sees as vicious uncivilized barbarians with no appreciation of anything of any value and whose traditions are abhorrent. She has an army of former Eunuch slaves, EACH ONE of whom is trained by killing a newborn baby plucked from the mother. So vicious barbarians and baby murderers. And she was carried to Westeros by a faction of the Iron Born who have the worst reputation in Westeros as pirates etc. That is what she has. And three dragons. The men she surrounds herself with have little more to improve her image with. Tyrion, a kinslayer (one of the most heinous crimes in Westeros is to kill your own parent) and (popularly) a kingslayer. Jorah Mormont, a former slaver exile who had been condemned to death by the famously honorable Eddard Stark. Theon Greyjoy who is hardly respected by anyone in Westeros and betrayed the people who raised him. Not a single person of honor seems to follow her or align with her. This problem was not faced by the Mad King. He still had people like Arthur Dayne and Barristan Selmy serving him. Dany did have Barristan for a while, but even he is already dead and cannot speak for her. Even the Citadel Maesters (normally a rather skeptical bunch) seem to have the worst expectations of the Dragon Queen. On top of this she has to deal with the fact that the country’s most recent memory of Targaryen rule is not the centuries before her, but that of her own father, the Mad King (on the basis of whose name she is trying to claim the throne).
Cersei has used these facts to vilify her quite well. It explains some of the Tarly reluctance to bend the knee to her as well. Tyrion’s initial idea was legitimate (i.e. to use Dornish and Tyrell forces, rather than foreign forces). Westerosi nationalism will not be offended by Westerosi forces and banners. Destroying central cultural landmarks like the Red Keep (which has been the seat of government for centuries) will only solidify this narrative further. The foreign conqueror who destroyed cultural symbols etc. will only unite the country behind Cersei further. In such circumstances, she could try to take the city, but will not be able to hold it for long anyhow. Once the people hate her and her foreign armies, they’ll give her guerrilla insurgencies far worse than the Harpy she faced in Meereen. And she’ll face this not just in one city but all over the continent. Against this, dragons are useless. Her Dothraki and unsullied won’t be safe unless they are camped outside the cities (and who’ll run the cities then?) and so on.
So Cersei’s strategy of cutting out her Westerosi allies first and foremost was actually quite clever. She has managed to exclude Dany from the narrative as being another Westerosi power player and turned her into a foreigner who will destroy the Westerosi way of life and hence created much nationalist sentiment against her. In modern terms, Westerosi people are seeing her as someone who is trying to create regime change on the basis of her superior but completely foreign forces. As we know from modern examples, once this sort of perception sets in, people will often even unite behind terrible leaders to fend off the foreign threat first. Seeing this, Cersei’s demand that Jon stay away from the fight makes excellent sense as well, since if the North allies with her, Daenerys can get some much needed Westerosi credibility.
When it comes to these complications, I’m not sure Dany understands the situation too well. Back in Meereen when Tyrion first met her, her perception was quite simple. She thought that having large armies and dragons would win her the throne. Tyrion pointed out the error of the view to her even then.
It is worth noting that the Targaryens were only able to conquer six of the seven kingdoms (the North capitulated without a fight to avoid any casualties) and were never able to conquer Dorne even with Aegon the conqueror’s large dragons because the Dornish used the same guerrilla tactics and abandoned castles etc. whenever dragon attack seemed imminent. They tried a direct land invasion under King Daeron 1 Targaryen. Daeron lost 10,000 men conquering Dorne and then lost 50,000 more trying to hold it after massive uprisings that eventually threw Targaryens out of Dorne. Dorne actually has lesser numbers than the other kingdoms. In terms of numbers the Reach may have the most. In terms of area the North is as big as the other six combined. Dorne was only finally brought into the Targaryen fold via marriage. This was probably what Tywin was talking to Oberyn about in an early season 4 episode about how Dorne managed to avoid the Targaryen fold for 200 years.
I’m not sure Dany has full appreciation of these complexities as Tyrion undoubtedly should (his book version knows it all beyond a doubt as he is quite erudite in terms of history).
The show has also done a bad job of showing what Dany actually holds for now. She doesn’t hold the Crownlands (Cersei does), the Iron Islands (Euron does), the North (Jon does), the Vale (Littlefinger allied to the North did), the Westerlands (even though one castle, Casterly Rock was taken). Does she hold the Riverlands or the Stormlands? There is no clear sign. And she keeps saying that she lost Dorne and Highgarden. Yet she keeps saying that Cersei will retake “half the country” if she goes North. But which “half” of the country does she actually hold? The North is half the country in terms of area and she doesn’t hold it yet. Has she regained control of the Reach? Samwell was at the Citadel which is in the main city of the Reach an the Maesters certainly didn’t speak like Dany was Queen there. In practical terms it still seems like Cersei holds the entire country other than the North &Vale and the island of Dragonstone.
So under these conditions, I do not feel Tyrion’s plans have been bad. Dany’s greatest advantage is in open field battles (like the loot train) where her forces and dragons can be used with full effectiveness while causing little resentment among the population at large.
Maybe the alliance with the North and possible marriage with Jon with help her with some of these problems.