"a good way to check if an NPC will be datable is to..."
I don't understand.
Blake - loaded savegame of day 160 - is in the relationship tab (no heart). But neither Gunnar nor "Cocky Guard" do appear in that relationship list - I'm pretty sure my Player Character met him in the very beginning at the "Track". The reason that he's not in the list - at that time - may be that my PC rejected him for quite a while.
On the other hand side Julian (owner of the Inn in NC) and Vicoria (River's Cave) are in the list. I guess both are not and probably will never be datable.
However, the game lists Gunnar when I load a savegame of day 194. Both savegames (day 160 and 194) are made on version 0.47.5.Extra.
So if Gunnar is datable in 0.47.5.Extra then shouldn't he have been in the list when I met him first?
I originally thought that any NPC that can be interacted with (more than fight) appears in that list when you met it for the first time. That would make sense since showing the value of "Like" and if there is a heart or not for any NPC might be useful. Doesn't it have an influence on dialogue options and events? However, Gunnar and Garuik were not added when my PC met them first so it does not work that way either.
Now, I might have misunderstood your reply.