I wouldn’t sell it. You’ll always be able to get a new car, but you may never be able to get an e46 m3 again if you sell it

Installing the down pipe vbands improperly is a known mistake people often make on these, so yes. Absolutely.

3M panel bond epoxy. It’s expensive but it’s for exactly this.

You definitely want two people. You should be able to remove the hinge pins and the catch. The electrical connectors probably have a plug just inside the door itself you can access by pulling away the rubber housing there.

Looks like maybe an e20 socket you’d need for those bolts

My 180hp z3 is way more fun to drive than my 450 hp 135i