3) 35mm lenses are fine to use because of the slot image (you donīt need an image circle with full coverage). Other mounts than Nikon are possible but will be a custom modification with a price tag to it.
Just a suggestion, maybe put a Canon EF mount on it by default? EF is an almost 'universal acceptor', then you can mount any of Nikon, Pentax, M42, Leica R lenses. The more available lenses the more popular it might be. With a Nikon mount you can only really mount Nikon lenses (and MF goes with everything of course).
An awful lot of electrons were terribly inconvenienced in the making of this post.