I would love to see Ryan get primaried to oblivion. That said, Trump has zero say as to who chairs the convention. His majority of delegates have 100% of the authority to pick the chair of their choice.
It would make for great TV if Ryan was named the temporary chair to get the ball rolling, then get fired, then get replaced by Sarah Palin.
A leader among Trumps delegates could orchestrate that.
I posted this to someone else, but Ryan himself said in May he would step down if asked by Trump:
http://www.cnn.com/2016/05/09/politics/paul-ryan-donald-trump-convention-chairman/