We have been to Valencia before in this circumstance, in 2013, despite all blandishments from JL to race him a then very young MM took the points and the title. It is going to take a mechanical basically, or a very strange riding error, there aren't the riders and bikes out there who can restrict MM to worse than 10th however hard they try to block him particularly since he is entirely likely to be starting from considerably better than 10th on the grid.
If MM is running 4th in the later laps I doubt there would be any requirement for JL to give up a prospective win at Valencia, which is a great track for Jorge but as you say not so much for Ducati. I don't think he would have regarded it as cheating to give way to Dovi at Sepang if he did, this would actually be hypocrital given his attitude at Motegi 2010, but as you say he may well think another chance for a Ducati win for him might be a long time coming, with such a win alone meaning he would surpass Rossi at Ducati which could just possibly be a motive for him, and the chance for Dovi for which he has made the sacrifice (again if he did) being rather a slim one as I have said. I agree it is entirely possible for Jorge to have worked out how to be faster than Dovi in an individual race particularly in unusual conditions, he is no slouch and has had nearly a whole season to do so.