See above re punting a year, and yes I think it’s important.
Yes, there’s still a chance he won’t be able to play 3B in the majors. If he doesn’t succeed there he can be at 1B instead. But obviously he’s more valuable as a competent 3B than a 1B.
Your injury scenario doesn’t make any sense, it’d just delay his call up.
If he was called up now, you’re giving up on him playing 3B, while wasting service time this year as a (negative) bonus.