Even if Wallace can play LT.... how well do they think he can play it. Is he just serviceable there or is he a legit asset in that spot? If he's even serviceable at LT then it's a good find for the scouting team to identify that out of him, but do we really want a "serviceable" LT. If he can be serviceable at LT but very good at RT, then I'd rather just leave him at RT.
I'm good leaving Onwenu at RT for now because he's serviceable there, it's a harder position to fill, and we have better IOL depth/development potential than OT on the roster overall. But he's not great at that spot and in the interest of this team fixing their issue of not having any great players, it's probably best to eventually transition him back inside to RG where he can be one of those great players.
I'd develop Wallace as a swing tackle to start this year and through spot playing time and practice reps, let the coaches get a sense of if he has high end traits that make him worth moving over to LT. If he doesn't show HIGH potential at LT, then just leave him at RT and try to fill the LT spot again next year.