A very, very interesting idea. I certainly see how it could work. Hell gets a few hints - it seems to be a place made up a lot of nasty spirit beings but with likely many individual players, but they all heed the largest entity. Like a criminal underworld, some beings are stronger and some are weaker but the biggest crime organisation is the one that keeps it all together. Like rival gangs but no one messes with the most powerful gang - the Fallen.
Perhaps Drakul resents the upset that was caused by the arrival of the Fallen.
Also, it is interesting that Drakul and Demon Lord are both mentioned are discussed in Dead Beat, a book about necromancers and even vampires (with the shadow of Kemmler too), but also in the same book is Mavra who works directly for Drakul and next appears in Battle Ground with him...using necromancy. Too many coincidences. Certainly suggests Drakul is at least involved with the Black Council but not necessarily working for or even with Cowl. The Demon Lord is an interesting reference in that book. Could be just for flavour. But what if it wasn't.
However there are some problems.
Almost every being cannot cross a threshold uninvited. Fae can if they have good intentions (but this could be related to their special links to humans, and their partly human nature). Angels seem to have no issues (not Fallen), or perhaps only Archangels, but they also don't seem to exist in the same time-stream either. But otherwise most spooky things seem to have an issue one way or another. This seems weirdly tied up with the extreme importance that almost all supernatural beings have around rules of hospitality. The noted exceptions are Outsiders who both don't care and have an obvious metaphor keeping them out of Reality, and the Old Ones are thought to be strong enough to cross thresholds like they don't matter and still have an awful lot of power.
Curiously, the constant references to Hell around Drakul could be a bit of hint. Not only that, the Old Ones (who rule the Outsiders) were considered to be a race of Demon Gods who were locked Outside (or in some cases, deep in the earth). So we know that Demon Gods can be pretty powerful. Not only that, some of the only other beings that disregard thresholds are gods. And the Old One "the Sleeper" even created it's own "holy" ground.
I don't know that I am yet totally convinced Drakul is a demon lord. But it's intriguing and definitely worth more discussion. Certainly, the author makes good cases for why it's less likely for Drakul to be Lucifer (the quote seems to outright discard Drakul as a possibility), or for Drakul to be a Dragon (again, the quotes imply there are only two Dragons left) unless people think Drakul is Ferro or Pyro. However the grammar implies Drakul is a separate being to either of them.
Drakul could very well still be an Outsider. An easy answer to why Drakul is allowed on team reality (in that scenario) is that he switched sides. The devil you know. Why an Outsider would switch when they apparently all work together is unknown. But it could be Drakul wishes to rule on his own, or perhaps he is a really long term sleeper agent. But in saying that...I think it's starting to look more like he genuinely wishes to rule. Which the Demon Lord thing would make more sense.