A: There are also shailohs since many hold that their marriages were valid, but their divorces weren’t, and children of remarriages are mamzerim.
B: again can’t that be the case with anyone? Who knows what happened down the line?
Although it is possible for there to be an unknown issue with someone’s yichus,
we assume everything was done properly because those involved were part of
the observant community which usually handles these matters properly.
The same cannot be assumed when the people in question were known
not to have been observant.