dark elixir drill inside walls?

Mine is out and very rarely gets attacked, seems like a waste having it inside

I would go for outer layer of wall if possible, maybe a tesla infront of it or something weak like a cannon/archer tower

If deciding between a gold mine or elixir collected, definitely!

Definitely inside. But lvl 1 is useless, get it to lvl 3 as soon as possible.

If there is an extra few spaces then yes. If not, keep it close to that walls and protected ;)