It is always best not to leave any applicable information on these types of forms vague or blank, if you are required to answer the query. You'd be better off to try to track down a common friend, an old letter or even your ex himself to learn the exact date. If that is impossible, then you'd be better off providing a guess and meanwhile finding out what you can and explain more at the interview. And yes, they could certainly ask about your former spouses, since they will want to make sure you are marrying for the purposes of immigration fraud.
This advice does not form an attorney-client relationship and is merely informative. It should not by itself be relied upon to address a legal concern.
Yes of course they may certainly ask about previous relationships. Your series of questions after the submission are making me wonder if you had adequate knowledge or assistance in the preparation of the application. One mistake many make is thinking that this is "just forms." These forms are the first step. Other more serious issues could be lurking unaddressed.
Do yourself a favor. Call an attorney to see if you may need an attorney to accompany you to the interview.
Dhenu Savla, Esq.
This answer does not create an attorney-client relationship and is not meant to be relied upon as legal advice.