Complex sentence: Each exiled prince that went to Pleskov could be proclaimed prince there (if principal throne wasn't occupied by other prince) but in any case he could get honorary reception and live there without fear for his life.
Split into simpler sentences: Each exiled prince that went to Pleskov could be proclaimed prince there (if the principal throne wasn't already occupied by other prince). In any case, he could at least get an honorary reception and live there without fear for his life.