I like this theory - although I do not have a sound proof as well, it seems to explain several things. So here are some more hints to support this theory:
1."And this is Bronn, a sellsword of no particular allegiance. He has already changed sides twice in the short time I’ve known him, you and he ought to get on famously, Father.”
A changing sides sellsword? Tywin saw in that a useful trait... Well Tywin needs Tyrion, but have no trust or love for him, so a close spy would be very useful.
2. Tywin has seen Tysha, so he knows Tyrion's taste in women better than Bron.
3. Shae wanted to be brought to court in spite of Tyrion's fear and warnings of his father's wrath. She's not stupid, so why would she like to risk herself that much? I can see only one reason: she knows that she is safe from Tywin's rage... because she is his spy.
4. Shae seems very interested to be involved in Tyrion's business as the Hand, and not only to be his whore. Does she love Tyrion so much that she wants him to share everything with her? Her testimony at Tyrion's trial shows otherwise.
5. We know how Cercei treats with "Tyrion's creatures" - she tries to kill them, and does not try to take them over to her side. So it does seem that Bron's title and marriage come from Tywin, whose favorite tactic is to buy the foes that he can use. Or was it a reward for a good service - spying on Tyrion?
6. When Tyrion finds Shae in Tywin's bed, she wears the golden chain of the Hand, a symbol of Lord Tywin's highest title. I can't imagine Lord Tywin Lannister allow a casual whore to wear this chain. However, if Shae is his faithful spy and courtesan, that's another matter.
7. Shae sounds very eager during her testimony - eager to please; but it seems more than just trying to please Cersei - it sounds more like trying to convince her master (Lord Tywin?) that she has not enjoyed being with Tyrion. Even if she knows that she is safe because she works for Tywin, Shae is also his lover, so it is important for her to show him that she thinks that Tywin is the true "giant of Lannister"
