Miguel, it has been said that Proto-Uralic (if it even really
existed) did not have much in the way of plural inflections. I'd
wager that the disparate plural inflections of the Uralic languages
arose at various times after separation.
Also, I'm not exactly sure how accurate Hungarian is in providing
grammatical information on Proto-Uralic. Obviously it has gone
through a relatively extensive re-analysis of forms, at least in
nominal morphology. Also, it's had extensive contacts with other
language groups during its history, so that might be a factor too.
The Mordvin stative seems related to the Finnish passive suffix -ttA.