I'd like to point out to anyone who thinks Euron has a teleporting fleet that in fact, it all makes logistical sense:
Euron's teleporting fleet: The ambush occurs on their way to Sunspear, not in Blackwater Bay like many believe - Ellaria says to Yara "WHEN we reach Sunspear, I'll treat you to a Dornish Red". We do not know for certain that Yara's fleet left at the same time as the Unsullied. I'd say the Unsullied most likely left before Yara did as they have much further to go - so no reinforcements. I don't recall anything that suggested Euron's entire fleet escorted their new prisoners back to King's Landing. He went alone whilst the rest of his fleet sailed for Casterly Rock. It's not unfathomable that his fleet caught up to and overtook an unsullied fleet - after all, they're Ironborn while Unsullied are foot soldiers... Euron was not present nor was he required in the West.
Lannister Army vs The Reach: Let's be honest, Show or not, Book or not, the entire Reach is scrambling with no leadership, especially after Tarly defected and would have stood no chance against an organised Lannister army. A small organised well lead army would decimate any larger, leaderless, unorganised army. Besides, it's not as if the entire Reach was there to defend High Garden. Tarly would have likely mustered and defected with a significant portion of that army also. In fact, he could have been playing both sides, allowing Olenna to believe he would defend the Reach for High Garden and then simply defect once they arrived.
Defenceless King's Landing: Cersei is Tywin's daughter after all, she's not stupid. The fact she's Queen right now suggests so. Any bit of reconnaissance would have revealed the bulk of Dany's army sailing away from Dragonstone. And, she likely took the risk of knowing Dany would not simply sack King's Landing as she is not the type to want to be Queen of Ashes. No defence - or at least a large one - required.
My problem is that they've improved the balance of power by strengthening Cersei, but in the process, they've turned Tyrion, who is meant to be known for his tactical nous, look like an absolute amateur. They've done a complete 180 on Jamie's character development which frustrates me more than anything else as his was one of my favourite in the books.