Didn't Pence at one point officially endorse Cruz, but in the same interview explain why Trump was better? Maybe Trump can do the same here. Officially endorse Ryan and then give a list of reasons to vote for the other.
Fox reporting Trump to endorse LYIN RYAN tonight I am NOT HAPPY about this LYIN RYAN NEEDS TO GO!! Please Mr. T DONT DO IT!!!
Yes, Trump said it was the best non-endorsement he ever got. Mike Pence endorses Ted Cruz - CNNPolitics.com - April 29, 2016
I'm not going to sepnd much effort speculating on the various possibilities - he'll do what he is going to do, we'll know what that is by 6 hours from now.