I have only read a few of the novels and watched the anime, so my knowledge of the series is not as good as I'd like for this kind of revision, but I'll give my opinion from what I've read/seen. Celty is the easiest, so I'll start by her.
Celty is nowhere close to MCB. The most destruction we've seen her do was in her berserk form towards the end of the first part of the novels, where she turns into a ball of shadows and rams through multiple walls. The feat listed on her profile, extending her shadow over multiple city blocks, is clearly a range feat, the only thing she has done at that range is restrain dozens of people. Her Durability is also clearly not that high, since she could be vivisected without much issue for the surgeon and she used a shadow wall to defend herself from an antimateriel rifle. Powerscaling would put her as Wall level physical AP and Durability, since as a Dullahan she should be on par with top Durarara humans and other supernatural creatures such as Ruri or Kasane. Anything higher, like the time she stopped a helicopter and its blades, was achieved only through her shadows. Her reaction feat of slicing several bullets and catching an RPG with her shadows is anime only, in the novel she just makes a shadow wall in front of her to protect herself before being shot.
Shizuo is a bit more complicated. Several of the feats noted on his profile (tanking dynamite, destroying a building/building-sized explosion) are things that didn't happen in the anime. His dodging of gunfire is also fishy, since the only times he has done that in the anime he had a clear line of sight on the shooter so it's likely aimdodging, not to mention the novel mentions Varona was using subsonic bullets. The pens feat doesn't make any sense, the durability of the pens has nothing to do with their AP, which would come from the person wielding it, a regular human who managed to use them to pierce his hands. That's a lot compared to normal knives, which barely pierce a centimeter or two of his skin even wielded by someone like Izaya, but hardly above Wall level.
Now, I've only read some of the novels, so the dynamite and destroying building feats might be true, but the anime doesn't show anything like those feats. While the anime changes minor stuff and even embellishes stuff (boosting Celty's reactions and giving her new powers), I don't see it removing that kind of feats. Thus, I'd suggest for those feats to be removed until a more knowledgeable fan can provide an actual source for them. Meanwhile Shizuo should be downgraded to Wall level while casual, Small Building level when serious since he's undoubtedly the strongest character in Durarara by far, as recognized both in-universe and by the author. His notable attacks section needs some work too, he's only immune to Saika's mind control due to the weaknesses of the blade (noted on
Anri's page). The shadow gloves are also a one-time thing Celty gave him, not standard equipment.
Izaya and Simon should be Wall level, the two struggle against casual Shizuo, and Izaya can only barely hurt him with his knives and admits that a serious punch from Shizuo would kill him.
As for the second question, it's pretty clear that Durarara, Baccano, Vamp and Etsusa Bridge are stories set in the same universe, so they are technicalle the same "Naritaverse" and scaling between them should be allowed. This is important since Shizuo apparently fought a Baccano character that might upgrade his speed (and thus Izaya's and Simon's). This is both stated in Durarara, where Shizuo mentions a weird foreigner that gave him a good fight, and in Baccano where said character is mentioned to have visited Ikebukuro in a vacation.
