Maybe the Behemoth was meant to be (or once actually was?) a battlefield summons? There are numerous erroneous descriptions that were written years ago.
As far as #2, I'd say go try it. But a few things - first I am pretty sure that all poison is time release, so if someone is hit for 5 from Foul Vapors, will probably take 1/turn for 5 turns. Also bear in mind that it will likely not stack up enough to kill anyone, as the BE will end once the caster leaves the field. And the skellies will dissolve immediately as well, though they aren't much needed because the BE will be over anyway, and the remaining poison damage will tick out at resolution as well.
Could be interesting if you had a high Death pretender, and put a Shroud on your Marshmaster, so that you are at least spreading numerous afflictions around the army. This will still be much more effective if you can at least bring bodyguards to drag the fight out, so you could script Foul Vapors + Holdx5 + Retreat. That much duration might even get a few kills, but you have to survive that long.