Carter, Lynagh, Barnes, Cooper all spent time at 12 early in their careers and were 10's. I would argue time at 15 has more of an impact on their future at 10, look at DMac.
No chance of him being a crash ball 12 with Kerevi outside him and CFS on a wing, plus Tupou and Timu crash it up in the backs of lineouts.
I disagree that he'd not turn into a crash ball center, look at how Paia'aua is playing with Lance at 10, he's being forced to play like a crash ball center. This is because Lance doesn't create that space like a Quade would and doesn't threaten the like a Foley, that's okay for some teams, like when we play Kerevi at 12. I feel like if we play Stewart at 12 outside of Lance he'll have no choice but to play crash ball rugby because he won't be getting the ball with space or time. The concerning thing is Stewart
can play like that and could be quite effective at it, but we don't want to be forcing him into that role at this point -- wait until we know he can't do those other things before we limit his game.
I do accept there are players who have made the transition from 12 to 10, but it's more that they were always going to play 10 just to get on the field they were playing 12 or it was as a duel playmaking role. If we were to play Stewart at 12 it couldn't be as a second playmaker -- we've seen how that works with Paia'aua. Stewart and Paia'aua at 12 would need to be catching the ball with time and space otherwise they are wasted. Lance really needs to be either a conduit to the players hitting it up, or the team needs to have the 9 doing the playmaking (like Genia at the Rebels).
Regarding 15 being a stunt to player development, I don't entirely disagree, I think though that's only really an issue if it's long term time at 15 without stepping into that 10 role, or when it's the ball-running/knifing 15. If Stewart were to play at 10 he'd more than likely be interchangeable with Lance at 10 Stewart would be doing some of the playmaking and would be likely shifting to 10 if/when Lance comes off.