I think Archer is the only certainty to move. Kela is more likely than not.
You're right that the rest of it is up for grabs. I'm also interested in the DH discussion and the Bell/Polanco race-to-not-DH. I've kinda accepted that if the team is going to sneaky-compete in 2021, Bell's left-handed bat has still got to involved in there.
The optimist in me thinks we're going to look at SP as a strength in 2021 highlighted by Taillon, Musgrove and Keller. I predicted as much!
I think that's right, although because of 2021, I am vacillating even on Archer. But I think in all likelihood, the big target for the front office really begins in 2022, so as long as Archer can bring back organizational help in a trade, he's absolutely gone, as is Kela unless he's extended or something.
Musgrove and Frazier are both possibilities, and I'd probably put Bell somewhere between those two and the Kela/Archer likelihood. I'm both a bigger fan of Bell than a lot of the regulars and a way bigger fan of Polanco, so to be honest, I'd kind of be looking at Polanco as the possible DH over Bell. It's certainly in the realm of possibility that Polanco could play well and be traded this summer, but I think the range of possibility for him is a bit more narrow right now: he will either prove his worth and be someone to stick around for at least a little while longer, or he will re-injure himself, struggle significantly with consistency, etc., and we'll stumble through one more year of his contract and then not exercise his option (or send him somewhere with his contract partially paid down).
In any case, re:this deadline vs. competing in 2021 or 2022, I think Bell is just more likely to go. There are also possibilities here, because Bell could still hold a reasonable amount of value in the offseason, or in the craziest scenario we could invest big in him as a face of the franchise. I'd put the odds of the latter at <5%, and that might even be being charitable. Frankly, I think the best possible outcome might be that Bell plays out of his mind this season and Cherington can have a deadline bidding war for him, with teams (particularly: NYY) gunning for a big impact guy immediately who also has some cost-ish controlled value going forward. If the deal isn't there, Cherington shouldn't force it, but big deals for Bell, Archer, and (to an extent) Kela would really sharpen up the "system build" or whatever we want to call it, positioning us to charge forward into 2021 with the core largely coming into focus (maybe you make a big extension for Musgrove and/or Reynolds, even Hayes, perhaps even Cruz in winter 2020, not really signing any FAs to speak of and going with the young team in 2021).
That's the hopeful narrative that's starting to come into focus for me, but there are lots of possible ways things could either go sideways or take a different route. To return to what I said at the beginning of the post, if the assessment is really that we can step up in 2021, then maybe Archer is even kept, but I think his value will be high enough to necessitate a deal, especially if he is able to approximate a front line starter who could be impactful in a playoff series. Honestly, if Cherington/Sanders would be able to get a deal similar to what they got for Stroman, then I don't think you can really hesitate. And, although Archer is highly disliked by lots of people, it's really hard to tank starting pitching value, so I think even a middling, 4-ish ERA Archer is going to be that valuable as a deadline acquisition.
Trying to sum up a bit, I think the possibilities for a surprise 2021 surge make Bell slightly more questionable to deal, whereas with Archer, you'd really have to have an analysis that you can win the playoffs in order to risk keeping him and then pivoting to a deal at the deadline if you are wrong. I'm more optimistic with the sight of baseball now finally on us, but that's a step too far, so yeah, long story short, I agree that Archer and Kela are the two definite deadline auction pieces.