Bummer.
I've not run across a Beretta single trigger that couldn't be made to work by the time honored soak and air blast that you've already done, but my experience is only with O/U types.
Beretta is infamous for wood chips from their machine inletting process floating around and jamming things up. I've had this happen with 3 Beretta O/U's that I've owned and heard about many others.
Did you try it for function (trip left, bounce the back of the frame on a piece of carpet, trip right) with the stock off?
That's all I can suggest without having the gun in my hand. Look for inletting residue.
You could hunt with it. The barrel selector is pretty handy to your thumb... just get your thumb out from behind the top lever before you shoot the second barrel.
