I really think if we sign Trumbo that he's the DH and we deal with Kim in the LF. I don't think they'd consider manning both corners with those guys again. I think Trumbo at 1B and Davis in RF is more likely than Trumbo in RF. Or at least I really want to think that.
It's just bookkeeping, but Trumbo is so bad in RF that his WAR probably goes up moving from RF to DH. Positional adjustment for RF is 10 runs higher than for DH. Trumbo's UZR/150 and his Rdrs/yr (the defensive metrics used by Fangraphs and BB-ref, respectively) were right about -10. I'm surprised he wasn't rated even lower, frankly.
I think you are correct, if we sign Trumbo he's going to be the DH and I don't like that. Not at all and here's why. That puts Kim as the everyday LFer which isn't idea, but it also means that we are going to have some fringe player in RF.
Even if you put Trumbo at 1B and Davis in RF so that Kim can be the DH then you are going to have a fringe player in LF.
The only way it makes sense to me, if you sign Trumbo is one of the two following scenarios.
1. You go ahead and sign a quality RFer, DH Trumbo, and leave Davis at 1B. I would actually like this but doubt they would move the payroll up that much.
2. Have Trumbo be the DH. Move Davis to RF. Play Mancini at 1B and see what happens. The bottom line being that you hope Mancini brings more to the plate than a fringe OFer.