1. I prefer
Dimitri, because to me
Dmitri looks like it's missing a letter. I really like
Dimitri though.
2. I haven't heard any, and I probably wouldn't use any.
3.
Dimitri Feodor,
Dimitri Pavel,
Dimitri Pascal,
Dimitri Radomir,
Dimitri Spirodon,
Dimitri Valentin4. No.