I've found out my CSV contained RRULE records separated by , comma, not by semicolon ;
That ruined deleting any of imported events.
I've corrected the CSV and now events do delete fine.. except... if I got RRULE repeatss set and I delete all events.. I still cannot delete the whole calendar, if any of repeats do exist:
Got an error "deleting this will make events unconnected - delete them first" inspite of no events exist.
Reason: jevents_vevent table still contains the event i sucessfully via backend.
I guess deleting the events currently work dreadfully - it should check EVERY table and kill the selected event records by event_id, but it does not.