It'd be interesting because Schmeling never fought someone as big as Carnera who was top level. I'd pick him anyway, but Primo could give him some problems.
I really tend to underrated Carnera bcuz of his performance vs. Baer, who imo was not a great fighter, just a tough one with a good right hand.
If Schmeling is allowed to win he brutalizes Primo Carnera. At least boxes his ears off. The only real danger for Max would be if Primo fell on top of him. Carnera wasn't legitimately top level.
Carnera's size would pose some problems. He was better than advertised. However Schmeling had a great right hand and Carnera as we saw in the Louis bout and early in the Baer bout was easily hittable with them. I think Schmeling wears him down late for a stoppage.
Saint Primo uses his handspeed which surpasses light and travels into darkness to make clueless onlookers believe it is a fix when Primo doesn't visibly make contact and Max's head is blown to bits like he's been shot with a 12 Gauge shotgun.
Carnera wasn't the greatest fighter, buy he was very solid contender with some nice wins on his resume. For a brief moment he definitely was top level.
I'm pretty confident Schmeling would outfight Carnera and take the fight to him for 15 rounds. Even stop him but if Carnera somehow managed a decision over Schmeling I wouldn't be incredibly shocked but surprised none the less.