Here's how I think the roster/lineup should be constructed. I'll start with the two obvious ones: Aside from injuries or rest days, Xander Bogaerts and Rafael Devers should be in the lineup every day. Neither are sublime defenders, but they both provide more than enough offensive prowess to be firmly entrenched as the everyday shortstop and third baseman respectively.
The Kyle Schwarber predicament is what makes things difficult. I'm totally fine trading Aldo Ramirez for him, but not if we're not starting him essentially every day until the end of the season. The lineup construction is hinging on whether he can passably play first base. Here's the issue: when Christian Arroyo comes back, he'll likely slot into second base. I'd be fine with him playing more first base, as I think his injury was a bit of a fluke, but it's not his primary position.
Fitting J.D. Martinez, Schwarber, and organizing the outfield into a lineup is an immense task when Martinez is bad at every position besides DH. The Red Sox would already have a very established outfield when Arroyo comes back with Kiké Hernandez, Alex Verdugo, and Hunter Renfroe. They could have days off where Martinez or Schwarber plays, perhaps in lieu of Verdugo who is having only a somewhat good season.
As for the infield, I think Jonathan Arauz is the versatile infielder we were looking for in Marwin Gonzalez, and I hope he's here to stay when Arroyo comes back. I don't feel Danny Santana should play another game for the Red Sox, and I'm struggling to see a role for Travis Shaw beyond a stopgap until everyone is healthy. Bobby Dalbec has been better as of late, and has shown enough to at least be the choice over Franchy Cordero.
I still have hope for Jarren Duran, who was promoted with tons of expectations. I think a full spring training knowing he'll be on the roster next year would do him well, and I wouldn't be terribly mad if he were demoted again before the end of the year, but I love the idea of having a pinch-running speedster on the postseason roster.
Christian Vazquez is by far the most underrated player on the team. I have seen people blaming his pitch calling for poor pitching performances. His hitting hasn't been great this year, but hitting quite literally doesn't matter if you're good defensively, and Vazquez is. Plawecki rocks, but his defense is mediocre and I'd still rather be starting Vazquez.
Here's my optimal lineup
Kiké Hernandez, CF
Xander Bogaerts, SS
Rafael Devers, 3B
J.D. Martinez, DH
Kyle Schwarber, 1B
Hunter Renfroe, RF
Alex Verdugo, LF
Christian Arroyo, 2B
Christian Vazquez/Kevin Plawecki, C
As for the roster, I would keep all the hitters currently on the roster, a.k.a those listed in the lineup plus Dalbec, Duran, and Arauz. I would cut Travis Shaw for Arroyo when he's activated, and get rid of Danny Santana. Lemme know what you think!