Thanks for reaching out! DXMON's an interesting thing, in that it is and isn't a BXLLET system. So, no, you definitely don't need to make a BXLLET character first, though a character running both sounds like a really fun disaster waiting to happen.
We talked about including the dice mechanic stuff, but opted against it. You could definitely use BXLLET's dice system w DXMON, but you could also use a different resolution mechanic, or not use one at all. Very few of the games of BXLLET I've run or played in really even use it, lmao.
As for NPCs and demons using powers, that's a great question! NPCs for sure are paying costs for their powers, but I wouldn't worry about tracking that too much unless it's an important character or a prolonged scene. If someone gives up five years of their life and gets their head taken off seconds later, it doesn't much matter. As for demons, they can essentially do whatever they want. Like with Beasts from BXLLET, they're intentionally a bit abstract, because there's not really any way to know if you'll decisively win against one. That being said, the way we sketched out the demons in DXMON was intended to give a rough idea of what they're capable of, and the ways you could potentially counter or avoid them. The details will come down to the tone at the table, but just like in Chainsaw Man, demons are terrifying and unstoppable up until the moment they're reduced to a pile of gore.
Lemme know if there's anything else you'd like clarification on, and we're really glad to hear you like the game!