No no no, a perfectly in-character way in, no DR trick (I suppose you're never sure where you'll end up when you leave the Death Realm right?).
Not every vilain is a Black Flamer... yeah, gotta keep some privileges for the evilest of evils!

Well I can't imagine why a sewer system wouldn't lead outside the city, that would be filthy and stinky... It probably connects to a river, preferably a large one if able engineers designed it. And the Vigesimi office probably have plans of the sewers (and surely people that work down there) so... If a passage there is, a secret one it is.

You might want to contact the Black Flamers ingame in order not to interfere with what they've been doing and plotting so far, and not to introduce incoherence.

I don't think we could make a convention out of your system because it takes all the protagonists to have the same conception of what has a chance to succeed or not. The reason it works in a few tabletop RPG is you have the GM as an arbiter of every player's character action, which we can't decently ask in PS.

See, the problem is someone with a lower score has 0 chance of winning a skill contest here.
A way to do it would be adding the score of the stat to the roll of a D400 (or a D200 for a skill), higher score wins. This way, the higher score a more chances that the lower, but no-one is condemned from the beginning by the numbers.
In your exemple, A would roll his (Axe skill + 1D200) against B's (Shield Handling skill + 1D200). If B is higher, he blocks, if not he's hit by the axe.
Then if it's blocked you would have every opponent roll 1D400 and add his strength.

Hide the Creation infos and Factions infos concerning the Black Flame.

Well I really don't get the point... All you're going to have is people leaving because of your policy (ant not the acts it themselves) and other people crating Ynwnn and roleplayingand descibing them as Diaboli. So basically you have no result AT ALL but making yet more players leave.

Why make categories and names? Just make a part of every critter's stats random...

Well... If not intuitive, a RP will suck. So I believe a school is kinda pointless; there are no such things as technics in RP, or only minor tricks but not what to make a class.