Thanks for writing. No, we are not supporting 8K content at this time.
It is considered spam if you submit the same clip with a different frame rate. Pick what you think works best for your content and stick with that. Do not upload the alternate option please.
I can't answer the first part of your third question, you'll need to make that determination yourself. Regarding what resolution to submit, I recommend you submit 4K clips. If a customer wants the HD version that is a purchase option for them as the system will downsize it automatically. The reverse is not true of course.
Please review the upload specifications for video content for more detailed information here.