Without having both the tires and wheels in front of me, I can't tell you what is wrong.
What I can do however is say that plenty of people have mounted and run MUCH narrower tires than 255s on 9.5" wide VMR wheels without beads not seating. I'm talking 215s. A full 1.6" narrower of a tire.
Who did your work? Are you sure it was leaking from the tire bead and not the valve stem?