While I think the problems you mentioned are very valid, that's pretty much where it ends.
I think what pushes mega diancie to A+ are its unique STABs, along with its access to crucial coverage like earth power and psyshock.
Basically, its STABs, rock and fairy, make it EXTREMELY difficult to built around.
Most answers to fairies, such as cobalion, heatran, and talonflame, all get destroyed by mega diancie; you practically NEED a CB/AV azu or scizor to be save from this guy, and moonblast is still easily 2hko'ing non AV azu and scizor can't switch in b/c mega diancie like to run hp fire.
This is what bothers me the most; i can dedicate a slot of my team to checking fairies but i still need to stuff in another way to have a chance against mega diancie because 90% of my answers to fairies all get decimated by diancie, a fairy u_u
Then you have its god-like ability, magic bounce, which means it doesn't have to fear random stupid thunder waves or will-o-wisps or taunts or whirlwinds, meaning it can easily set up on skarm, mew, non eq gliscor, sdef tran, mandibuzz, etc.
Its natural typing makes it a pretty decent birdspam and charizard check, outspeeding mega pinsir and both zard forms, and resisting talonflame's stabs, while ohkoing everything mentioned with rock STAB (bar bulky zard-x which is dying after rocks anyway)
The competition argument is kinda dumb because although the megas you mentioned are a little more consistent, they fulfill completely different roles (none of them come close to checking talonflame for example); mega diancie is unique and nothing comes close to offering it competition; its a fairy that beats and outspeeds fires with magic bounce and 110 base speed. There's simply nothing like it.
I can agree that its not consistent and its a little difficult to use, but its natural typing, ability, and STABs are so devastating and unique that it should be A+
Actually few people run HP Fire on Diancie cus Earth Power is definitely better as it allows to get past of Heatran, Excadrill and Magnezone to make some examples. Hp Fire lets you beat Scizor is true, but EP is just more consistent against all the other Steel-type mons. Also, as I already said before Diancie has a very cool moverpool but it has to choice which moves to run. Moonblast and Diamond Storm are mandatory ofc, then you have to choice which moves to run beetween Protect, HP Fire, Earth Power, Rock Polish and Calm Mind. Obviously it can't run all these moves and it will be stopped by something, but that's for any mon I guess, and STABs + Earth Power are usually the standard offensive moves anyways. I would also to say that even if people (and myself included) usually prefers running Rock Polish or Calm Mind in the 4th, Protect shouldn't be understimated as an option on Diancie. You seem to have ignore the issue that Diancie has to mega-evolve but that is a really one imo. 50 / 110 / 110 defenses are bad for a tier where sweepers usually have 150+ attacks, also Diancie is very slow before mega-evolving and therefore even easier to kill. If you run Protect you avoid this, but ofc you are losing an important move so you better try to find a safe way to mega-evolve Diancie, but that's not easy as some people think. Also, it's true that Diancie has some features that some megas haven't, but hey Metagross, Lopunny, Sableye and even Altaria and Gallade to some extend are just better overall, so that's a point that doesn't favour Diancie either. However, what you said it's true and I can see why you and other people would see rising Diancie to A+ rank, and I agree on some points myself, I just wanna see what some other people think about that topic.
Happy New Year everyone, btw.