1) If there was no civil law precluding it, is there any reason non-Ashkenazim couldn’t be married to two spouses?
I once attended a wedding of a friend (who was marrying a Sephardi). When they read her ketubah out loud, I noticed an interesting clause that stated that he would not take another wife without his (current) wife’s permission.
I suppose such a clause (I don’t know how widespread it is in the Sephardic world) would do the trick.
The Wolf