With your rounded screw issue I cut a nice deep cut with a dremel, usually cutting into the surrounded area a little then get a nice wide thick straight screw bit in my torq driver and zap it out, the key is getting a deep enough cut. If you don't have a torq driver use a racket as if the screw was rounded it must be stuck in there good so a ratchet handle gives you better leverage.
Shame about the chassis, looks like previous owner bought a brace rather than a new chassis, but if it's holding strong no point stressing over it.