I think you're making an unfair generalization. George Martin has introduced so many characters and plot threads, that anybody can get confused. That's why there are online wikis about these novels.
That's exactly why they should have done a scene with Jamie and Tyrion where Jamie briefly gives the backstory, and tells Tyrion the truth. It would have taken what, five minutes?
But that's my point. In the book, Tyrion is full of rage and heartbreak. He's just learned that his cold hearted son of a bitch father pimped out a woman who actually, truly loved him (the thing he's ached for all his life), and he's going to make him PAY for the pain that caused him. In the...
The more I think about it, the more I think they blew it. Whatever Tyrion had done in the past, he was never one to kill someone in cold blood. Now all of a sudden he decides to do just that by killing Tywin in cold blood? It played FAR better in the book.
I've been greatly anticipating Dinklage doing these scenes involving his trial and its aftermath, and he hasn't disappointed me. He's been brilliant, and I'm anticipating what's to come even MORE.
So is Jamie's. He was the most skilled swordsman in Westeros, the literal Golden Boy, rich, powerful, his father's favorite, and had the woman the books described as looking like a "goddess". Now look where he is. Oh, and let's not forget the burden he's had to carry of being labeled...
In the beginning, Arya wasn't a cold blooded killer who would gleefully stab an unarmed boy laying on the ground, which is also a change in kind. Have you lost your investment in her too?
If you haven't read the books, this won't be the last time you'll be faced with a "change in kind".
BTW...
Her words said no (weakly), but her body language said yes. This is a man she has lusted for ever since puberty.
My investment in the character wasn't curtailed when he tried to kill an innocent boy, and it isn't now. I never expected him to be noble and chaste.
I don't understand all the outrage about Jamie. He's become more sympathetic, but he's hardly stopped lusting after Cersei. After all, it's not as if he was raping some poor young innocent. Her only real objection (half hearted at that) was the time and place, not the act.