The thing is Potatoes said he cannot go to medic 4 times and the guy straight up went "fuck you and your orders I'm going to heal anyways." That's a form of rebelling. He knows that he isn't following the orders and that he's probably going to get shot or die for it. If Potatoes only said no once and he went for it anyways then that's another situation, but Potatoes clearly states that he said it multiple times to the point where the guy acknowledges that he said No, but he just didn't care.
With the no past first cell thing, yeah if they take a step past it, they get warning shotted. Just like if someone was 1 step out of the weight training room, they'd be warning shotted, but if the guy ran to like soccer when it was no past first cell, he'd be considered rebelling. Just like how he ran to medic when the orders were in weight training room.