The only reason Betts would be locked into RF is because JBJ is the better defender between the two. If JBJ gets dealt, there's no reason Mookie should stay locked in RF in deference to Benintendi; my feeling is that Mookie is probably the better CFer and CF defense is still more important than RF, even in Fenway.Benintendi is a CF and would move over to replace him.
I don't necessarily endorse trading JBJ, but I think Betts is locked into RF regardless.
If Benny can't handle RF in Fenway, then he looks like the more obvious trade candidate.