I am becoming convinced that the show is setting things up so that
Tyrion kills Tywin next season, but not Shae.  I still think she will
agree to testify at his trial with the intention of hurting Tyrion,
because she believes he has fallen in love with Sansa.  But at the last
minute she changes her testimony to defend…

He made Roose Bolton Warden of the North in exchange for Roose greasing the way for getting Jaime back to King's Landing.

I think the relationship between Tyrion and Sansa will grow stronger at the beginning of next season, pre-purple,and that will push Shae into a spasm of jealousy, leading her to testify against him.  Shae may even believe that Tyrion killed Joffrey, out of love for Sansa, to get her some revenge for Robb and Cat. …