Does it still leak with the valve set screw out?
In on of my MQ2 markers, it had a slight leak...so I let it sit gassed up overnight. The next morning everything was "seated" and the leak was gone and hasn't reappeared. Also...if you wind up taking it back apart, a little nail polish or blue loctite on the poppet face will keep the screw from backing out and causing the same issue.
I have nothing for you on the runaway gun, unless maybe it's a trigger filter setting? Is it a very short trigger pull?