You can marry anyone…Not any NPC, just mean any male or female.
So obviously some NPCs are off the list. I assume I can’t marry Jarl Balgruuf, but what about Irileth? What about the Graybeards? What about already-married NPCs? Is it any “important” NPC, or is it more or less than that?
Has the Skyrim Creation Kit shed any light on how to determine this?
NPCs who can be married will be part of a faction, marriage-faction.
The console command
addfac 19809 1 while having an NPC selected will make them eligible for marriage.
For scanning through info in the Construction Kit, look out for
and the faction ID