Its all about making the note nicer. I know when my A6 finally needs a new exhaust I'll definitely be getting a stainless steel one. But then again it makes sense as I'm keeping my A6 for life.
I believe you might find the cat might be protected by the engine undertray. If not you can get cages that go round the cat as well as special paint to help it more identifiable
Airbag warning light issues are often caused by a loose wire under the passenger seat mainly. Can sometimes be drivers side. But definitely more passenger side.
Welcome to the forum you'll find the members on here are a friendly and helpful bunch 🙂 my advice would be to try and get a second hand unit in eBay. I doubt you'll get the button on eBay but you never know
I think you might be better off taking it to an auto electrican. The wire under the passenger seat could just be loose and would be the easiest thing to check.