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A long time ago at RPGDot we even had a Gothic fansite.

I've played Gothic ages ago, so take my answers with a grain of salt:
1. I think you don't need anybody from the Old Camp for the rest of the game, but it is possible a couple of people are still invulnerable. AFAIR you'll get a teleport rune to the fire mage house later. Maybe you want to wait until then.

2. There is no key for this door. You can find quite a few unfinished details if you run through the world with open eyes. The devs were short of time.