Is there a way of getting successive marriages listed in the right order (chronologically), even when I don't have marriage dates for some of them? I could just guess some dates, but I'd rather not have to do that.
Unfortunately not. Couples with no marriage date show last.
https://www.familysearch.org/en/help/helpcenter/article/how-do-i-put-spouses-in-order-by-marriage-date-in-family-tree