The normal order of things went much too slowly, so i sped things up by changing dwarfs to CHILD:2. Question was if dwarfs will marry distant relatives (cousins in this case, but aunt/nephew and uncle/niece should be the same as long as both parties are unmarried and of compatible age - remember that thanks to the practically endless fertility, an uncle the same age as or younger than his niece is quite common). They will. Cousins can be friends or lovers of each other, shown by "made a friend/talked to a friend" and "is romantically involved with" thoughts. They'll keep listing each other as "cousin", however. Probably because "cousin" comes first and apparently supersedes other relations. Once they marry, status will change to "husband/wife" (which happens to be the highest-priority relation a dwarf can have). I stopped the run after the first couple married.
Result: ^_^science :3 says you only need two married couples to repopulate the entire dwarven race.
I tried getting my hermit closer to her sequestered husband and see if that could result in reproduction. Setup was quite simple:
###
RFH
+F+
###
R = Rith, the renaissance single mother
F = carved fortifications
H = Hapless Husband
Neither got any "talked to the spouse recently" thoughts in about two weeks. Rith also didn't get pregnant, but that's inconclusive.
I couldn't be arsed to force longer vicinity, Rith has better things to do. It seems that being separated by a fortification already blocks social interaction.