(Continued. Evidently there's a 1,000 character limit on profile comments.)
If that doesn't work, it might be worth hitting the latch area with a torch to hear things up, expand the metal, and hopefully loosen things up. It might be difficult to get heat exactly where you need it in this situation though. The heat will definitely burn up the paint in the area where it's applied also, so I'd use that as a last resort.
If all else fails, and you have to cut a hole to access the latch assembly, I would cut the hole in the side of the box, not the door. It's out of sight and could easily be patch up with a piece of sheet metal, some silicone, and some screws.
Sorry to hear about that, but I'm glad you were able to get the rest of them open!
Is the hole where the handle assembly was mounted big enough to get your hand and arm inside the box? If so, I'd try sticking a can of penetrating oil inside there and hosing down the latch assembly.
Whether you can get oil on it or not, I'd try pulling on the cable with a bar before I cut into the door or box. Loop the cable around a pry bar in such a way that the cable overlaps itself and holds itself still when you pull on it. Position the bar such that you're able to use the edge of the handle hole as a fulcrum to put tension on the cable directly away from the latch. Cut a piece of wood or angle iron or something to put between the sheet metal and the bar in order to spread the force of the bar over as much of the edge of the metal as possible and hopefully avoid denting or bending the sheet metal.
