The thing is, we're not considering ST for it. I got a Dawn Stone in a half hour (took a lunch break at work), but I had to import a max-effort Pokemon to get it for my run and the only ingame Dawn Stone is literally at the end of Victory Road. Neither one's conducive to a high rank based on the rules we set out.
What I do take issue with is Mawile as a C.
Location: Granite Cave (Mach Bike required). Admittedly, a bit of backtracking once you get the bike but it's not too long.
Type: Steel/Fairy, essentially neutralizing the Steel weakness and turning Poison into an immunity. Leaves Ground/Fire as weaknesses.
Stats: Good Attack/Defense, HP is a tad low, Speed is good enough ingame. Has access to an ingame Mega Stone that makes it hit like a truck, gives it Pure Power and radically increases defenses.
Movepool: STABs provide good coverage, and with a couple minutes of beach exploration you have its best STABs in Fallarbor (Move Returner, nobody really needs the Scales on first visit). Has access to Strength and Rock Smash as filler and to cover off those HMs.
Major battles: Does OK against Wattson with some Rock Smash fun, loses straight out to Flannery, resists everything Norman throws, then you get Mega access and it pretty much wrecks the final three gyms (Tate/Liza in particular are jokes thanks to Iron Head). Only potential problem is Skarmory and Altaria's EQ, but you should be able to hit the latter with Play Rough first. Team Magma unfortunately is a concern because Numel/Camerupt are perfect checks, but it can pretty much take anything else they throw at you. Extremely effective against 3/5 of the Elite 4 (Sidney/Drake/Glacia) and can even handle a couple of Steven's Pokemon.
Other things: Its Mega Stone is in Verdanturf, and you can get it on first visit. If they swapped Mawile and Sableye's games then I'd call Mawile A with an outside shot of S since you wouldn't have to deal with Fire/Ground types as much.
As it stands, I think Mawile can jump at least one tier, no?