I think homebrew is fine, if you can keep it balanced.
You should still let the players make knowledge rolls to get hints as to how the creatures operate. But yes, if you throw a vampire at them expect metagame. Make something up like an Ash-stalker or crimson breasted arach and you put more of that exciting uncertainty in the game.
I'd say write the stuff out, just in case someone is getting sore and thinks you're just editing stuff on the fly to screw them over.
There are guides in the Bestiary for making creatures. Remember, it is an art more than a science.
Haunts are fun, but the thing I always thought was lacking is that there really needs to be a set way to get a 'hint' on how to end them. Like some type of knowledge roll to remember some old story about a murdered soon-to-be-bride who had a ring of mithral, that became stained when she was scorned and killed. Spook keeps happening until the ring is found and the blood cleaned from it.
I'm not sure what you're problem with 'explanations' is. What do you mean? If you homebrew, custom up stuff, it usually solves most of the issues if you're clever. I mean, with modules there's always this issue of 'why are we doing this' that constantly needs to be answered. Making a compelling reason to make... just about any class alignment race and history go and do the adventure is difficult, and most of them are square pegs shoving into round holes. The advantage of homebrew, you can tailor the story to make sense for the players, and challenge them appropriately. The downside, the burden of writing/creating that falls to you.
I don't mind "metagaming." I use it to serve my purposes.
We fought this big talking wolf once.
All we knew is it was "a big talking wolf."
Werewolf? Worg?
Big. Talking. Wolf.
