Hi there!
* I believe you can come from any country, but (speaking in a personal capacity) citizens of Cuba, Iran, North Korea, Sudan, Syria, or the Crimea Region in the Ukraine may be ineligible because of US sanctions.
* There is an onboarding process in the UK in late August. There may be other travel opportunities but we haven't hammered them out yet. If you'd like to talk to me privately about your individual case, you can email tommyc@torproject.org. You can find my PGP key at https://www.torproject.org/about/corepeople.
* The fellowship begins in September, plus onboarding in late August.
* I believe that is general language (other fellowships may have more restrictive application criteria, for example). The guidelines in this instance include this post, the application page at https://foundation.mozilla.org/fellowships/apply/, and, for good measure, Mozilla's participation guidelines: https://www.mozilla.org/en-US/about/governance/policies/participation/