Hey Aran,
Just wanted to say I am so sorry to hear about your troubles. I've not been online much at all, but I saw this thread and just read it all.
I hope you get the possum AND the weasel/mink/fisher/whatever-it-is.
I'm sorry especially that your son has had to have this disappointment. I know how my dd is about her pets (a snake got in the cage and ate Frisky, her gerbil, some months ago) and I know she would be devastated. I hope your son is cheered by his new chicks.
I hope you get it, and you and your wife can get some sleep. Hope your ducks are going in well now too, since I hadn't seen anything else on that. Mine just got good at going in a week or two ago when I renovated their coop ... seems they like it a lot better now.
And you shouldn't have any legal problem at all disposing of those kinds of animals. The only issue MIGHT be firing a gun, if you are within city limits or something like that, but killing the varmints of those sorts should be ok legally.
Did you ever hear anything from your local wildlife officer if they will do anything for you?
Well, again, good luck getting them!!!
trish