Regarding JDM (or Betts) staying or going as it affects the ability to resign Rick Porcello, I think Betts will hold out for the highest possible deal , regardless whether it is NL or AL. Probably a major market team due to the economics of paying him.
JDM is a different case. His contract values are preestablished, although could be rewritten and enhanced. He will be 33 in the 2020 season, and only he knows how much the back is bothering him. It seems to be a current factor, and we'll see if it causes any issues in the Houston series. Muscle pain can be worked on and can fade away. If it is a structural issue (discs, nerve impingement, etc) not easily cured. Surgery may be a season missed. As primarily a DH, JD will only be a consideration for an AL team, and only a few could afford him (NYY, Houston, CWS) . I would think he wants to play for a competitive , winning team. How likely are the Yankees to add his bat and pay Stanton ? Houston, completing JDM's redemption as a dropout there, may be more likely. Boston is still the best fit and should remain his baseball home at least through 2020.
Porcello will need to weigh time/money offered from several places. DD will have to weigh where you get 175-200 innings covered vs paying the guys in the field, not the pitchers. This would be self resolving if the Sox system ever developed a legitimate couple in-house young starters playing for MLB minimums for a few years.