To me it seems the trades are developed as acting to be presenting the main narratives (to either Hellboy, or the BPRD, or be it the Lobster or Abe, Witchfinder, 19somethings or any of it) into sort of accessible or overseeable chunks, fitting together - more or less - where sometimes uncollected remainders fit into trades upon reaching such a point (like the frogs-thing with Trimpe both as Severin on it) - eventhough that story as a whole took years before it reached completion.
As if a trade can also be to bring about extra logics and sense to how stuff could get read.
Because for Hellboy both as the BPRD - or any of it - it might be very much a purposeful thing that THERE WOULDN'T BE BUT ONE READING ORDER NEEDED. As if it would be better - or good at least - that stuff could get read for itself, because perspectives or new takes on things might offer themselves up as how they might. On anyone's own good time. Any such wouldn't have to be bad, but all good more rather.
I believe mr. Mike has said himself a number of times how he himself would feel to be seeing new perspectives on certain stuffs - eventhough he would have thought it all up, to a very large extent at least.
Anyway, I'm guessing that the making of stuff into trades would be done so that it would be purposeful, without seeming conflictive or weird too much. And like Middenway says, with a little waiting here and there, with some puzzling and some thought, Dark Horse is capable of such. But then, that's what they do: presenting books or sequential installments to them, in a captivating fashion.
And also comics seem to dictate themselves into containing never too much all at once.
So logically stuff remains untold, leads getting left into needing tyings up, so naturally the titles would expand or take turns with needing all the room. Which only adds to the idea that for comics it will be cool to NOT be telling things too linearly or straightforward-like all the time.
Like maybe hinting at certain BPRD stuff in Witchfinder, or expanding on something Hellboyish in a BPRD title, because stories or realities or wonderments don't necessarily act only according to typical "superhero continuity". As adding any inventiveness or creativity to such would be good more rather than bad - or at least it would be allowed.
I'd think Hellboy etc. for being creator-owned - for being what it is - supercedes mere traditional comic book continuity: it is innovating it. By being nÚt just a row of numbered similarly-seeming books.
(A diversity or variety which will have to be allowed in comics, because comics never amount to being just similarly seeming books. Spider-Man comes in any size and shape, not in any way bearing much sequentiality but strictly. As neither does Batman. Or any other comic book title.
Comics are ruled by variety. Since comics amount to being but one giant messy fiesta of hardcovers into floppies into prestige format glow in the dark out of the box A5 to A4 to A3 and back again, in 4 / 8 / 256 or 1244526.8 colored black and white bookies, ever shifting. One ongoing variety of strict diversity galore.
Comics or either well vacuumed carpets MUST be the obsessive or OCD-ist's nightmare... Which must be why the gods invented them, together with stamp-collecting and dustbins in the first place: so people might learn to be finding their own way or take on things. "Wanna be neat? Just try it buster, I dare ya...")