So - has Bron already been bought by Tywin at this point? Maybe explaining why Shae will end up with Tywin instead of on the boat?
I always thought Bron was bought off by Littlefinger who worked with Varys. Then when Varys frees Tyrion they set it up so Tyrion sees his Father w/Shae.
Varys actually tells Tyrion "don't go up there, nothing good will come of it". I think he was sincere.
ETA: obvious paraphrase, looking for the actual passage in the book (no pun intended).
It actually went like this (just looked):
T: My bedchamber is up there.
V: Actually it's your father's now.
T: How far is the climb?
V: Let's go, you are too weak, there is no time.
T: How far?
V: blah blah blah be careful.
V: This is folly, your brother has given you your life back. Would you cast it away and mine with it"
I don't buy into the notion that Varys planned for Tyrion to kill Tywin (lots of discussion on this on other boards).