I think the hinges are the non-destructive way to go - if you can actually unscrew them. I can't remember if they are screwed into captive or loose nuts and if the nuts inside are loose, turning the visible screws may not release the hinges. But this diagram and the contents of new hinge packs suggest the nuts are captive.
Plan A is with the hinges detached and the bumper removed, you should be able to slide the whole boot downwards just enough to disengage the boot lock, without harming the cable to the number plate light. Then you should be able to undo the boot lock/handle fixings and remove the boot handle. If the lock is really seized, replacing the lock barrel may be possible, or you may have to replace the handle and barrel.
Plan B would involve taking an angle grinder to the boot handle and trying to cut away enough of it that the square shaft through the boot lock can be pushed out (well, in) and the lock turned with a big screwdriver. Again a new handle and barrel would be required.