Highgarden/Tarly subplot makes no sense

[Azmodael]

Forget about the teleporting armies. Forget about the jet powered ravens. Forget about swimming in plate armor.

Lets talk about Highgarden and the Tarley subplot.

The Tyrels were loyal to the Mad King. Lord Tarley himself was a general in king Rhaegar's Army.

Then his lord gets blown up along with his family in an act of heinous mass murder.

Then Tarley commits treason by declaring for the women responsible for this outrage and proceeds to help murder his former Liege.

Then he get's beaten by Dany, denounces her and decides to end his family line out of his loyalty for Cersei.

WHAT THE ACTUAL FUCK?

I can understand switching his loyalty - he'd become lord of the reach, the Tyrels already have no heir and instability will ensue plus Dany brought dothraki savages. So even though Cersei is horrible he has a lot to gain from his treachery.

But by the same token his decision to stay loyal to Cersei makes no sense. Dany IS the rightful queen. He DID fight for her father. Cersei DID murder his former liege lord and a lot of other people from the Reach. His family line WILL die if he doesn't submit. And he chooses to die. For what?

The capture of High Garden also makes no sense.

The show revealed it as a mighty castle with high walls, easily defensible regardless of how bad a military commander you are. The Tyrel army avoided major fights, even if ALL the other families from the reach joined Tarley Olienna should still have several thousand troops to defend the castle + endless amount of provisions for a lengthy siege. The battle of High Garden happened off screen simply because there was no plausible way for Jaime to win without heavy losses or a protracted siege.

[WeaselPie]

Unfortunately, I think the Tarly arc was included to reverse-manufacture a split between Jon and Sam, and possibly Jon and Dany.  

Sam is obviously in a position of considerably greater potential power and clout now, and I think it was always GRRMs intention for Sam to go on that path in the greater story.

I agree the handling of Randyll was clumsy, and the throwaway method in which Highgarden was handled was bullshit.

[Jabul]

I agree with what you say, but I think you are not harsh enough in your judgement. 

Some (probably most) fans of fantasy and science fiction believe in the importance of world building. The first few seasons of Game of Thrones gave us an interesting quasi-medieval society. Season 7 has made an absurd hash of this world. Characters act ridiculously out of character. The religious, ethical, political, and social underpinnings of the society are frequently ignored by the writers, with some throw-away lines here and there. Many lords and ladies, heroes and villains not only forget their values, they often don’t even act in their own interest. 

Lord Tarly and the associated leaders who pledge loyalty to Cersei are examples of this process, as you detail quite well in your OP. Do I agree that Tarly’s betrayal of his liege is plausible? Well, sort of. I can see that he would at least consider the matter. I doubt, however, that he would follow through. It is not even remotely reasonable to think that the other lords would follow him. For one thing, they would definitely remember Stannis Baratheon and his accusations. There can’t be any doubt about the conclusion they would draw: We swore an oath to a Baratheon king. What the hell does that have to do with the Lannister bitch? She doesn’t even have the decency to keep the name of the dynasty. Everything that this so-called “queen” says and does confirms the assertions of the former Lord of Dragonstone. Just about everyone in the kingdom now acknowledges that what he said about our “queen” was correct. If our oaths mean anything, we should do all that is possible to make sure that she and her many-times-a-traitor brother/lover are beheaded. 

Then there are practical issues. Daenerys has three dragons. The lords of the Reach have the word of the Torturer in Chief that the Lannisters are working on the matter. Why would the worst idiot in Westeros be assured by this? Even idiots might be able to count. The Lannister/Reach forces wind up marching across open country with one, exactly one, anti-dragon weapon. The leaders of this expedition have somehow forgotten about the Field of Fire. They are also unaware of the fact that three is greater than one.