Hellboy its its own thing. I've read upto (and including) volume 8. It isn't impacted by B.P.R.D. at all, as far as I'm aware, so yes - it is it's own thing. B.P.R.D., however, fundamentally requires some Hellboy knowledge so whilst it is its own thing as such, it still has that fundamental tie to Hellboy.
Hellboy is definitely Mignola's own. More so than any other series he currently has out - Baltimore, Witchfinder, B.P.R.D. - they're all affected by the influence of others. So is Hellboy, but I would think Mignola keeps a very, very tight hold on it.
I understand there's a reason for all the archaic languages, and I know they add to the atmosphere, but for me it puts me off a bit. It takes me out of it, it doesn't draw me in.
On the other hand, Roger and Ben talking about wearing pants drew me in.