I shot a CPA for years using BP.  Never had a problem inside the action.  Ever once in a while I'd remove the block and clean just to be sure.   

No problems ever.

Tatume
I have had 4 CPA Stevens over the last five or six years, all have been shot solely with black powder. The calibres were 40/65, 38/56. 45/90 and lastly my current 40/60 Maynard.
None of these rifles have given me a moments problem. My cleaning routine is two "damp" patches followed by one dry patch between shots. Final cleaning at  the end of the day consists of  about five damp patches ( till they come out clean) then a dry patch. This is followed by spraying WD40 into the barrel and a little into the action and passing a patch, wetted with WD49 through the bore and over the metalwork. 
To this day I have had no issues whatsoever with corrosion or action malfunctions. My rifle is fired on a daily basis, for long term storage however I would go with one of the heavier gun oils rather than WD40.
Mike.

I don't remove the breech block every time.  Just every now and then.  It's pretty easy to do.  No more difficult than a highwall.

Chris.

A little airspace won't hurt anything.  My breechseated loads usually have a full case of powder, but not always. Even so I've always got between 1/16" to 1/8" of airgap or so.  I once experimented with filling the gap with a felt wad.  It was pointless so I stopped.

Chris.