I think the DBX and basic BD manuals actually list the rule differently which didn't help matters. The stacking to a -2 penalty is only in the DBX manual, the basic DB one simply puts stench as extending your normal threat to 360 degrees.
As for the slam question:
I've asked on the FAQ thread on Jake's blog but don't hold your breath for any resolution there.
Google turns up all of one forum post by some guy who says it should apply, but giving no backup source or reasoning.
I've come around to the idea that it should apply, not because of the rule wording which I still reckon says no (a modifier relating to a threat zone from the target does not exist to be enacted upon by the stench rules) but because I can't see any reason for that part of the stench rules t exist at all unless that's what they meant it to do.
Wording in question, Stench:
'In addition, the player’s Threat Hexes always modify a test (up to the maximum for that test) if the modifier is listed at all.'
Slam
'-1 per opposing player threatening the hex you are in (ignoring your target, maximum of -2).'