After a lot of thought, I've rationalized half of the spotpass characters being able to S rank the mothers in the game. Here are my conclusions:
Priam - Not story relevant, so he's safe and can't cause any paradoxes
Aversa - We have no idea what happened to Aversa in Lucina's timeline. It's easy to say she was always destined to be defeated and retreat to the Wellspring of Truth and defect to Chrom's side, and it's possible this could have happened before Chrom dies because Aversa is follower and not a leader- she does what she's told, but she isn't the person calling the orders. Validar could easily replace her after she defects, given the time.
Yen'fay - We never learn what became of the Yen'fay from Lucina's timeline, so it's very possible that Chrom was able to recruit him instead of killing him at Demon's Ingle. There's no proof that Spotpass Yen'fay is the one from Lucina's timeline (he just says it's an alternate timeline where Say'ri dies but this could mean a different timeline entirely), but in this situation he can't be, because he would have memories of marrying his spouse and having a kid. We are also shown several times throughout the story that fate always wants to make the outcomes of the original timeline happen- this force could be what causes Yen'fay to wander through the Outrealms and into the game's timeline in the first place. It's a big lengthy explanation, but it's reasonable that Yen'fay could have fathered any of the children in the original timeline!
So these three will have S supports completely open, with no convincing or further explanation required. The other three, however...
Walhart, Gangrel, and Emmeryn are all supposed to be dead. While you could argue that they survived like Aversa did, it's much harder to do so and their role in the story throws a wrench into things. Emmeryn's death is so central to the story that it quite frankly makes no sense that her Spotpass iteration exists. In the original timeline, because the way she died was via assassination and not a noble sacrifice, the first war with Plegia against Gangrel stretched on for much longer, and ended with much more bloodshed. I cannot see Gangrel surviving that after everything is said and done. Lucina also makes no mention of Walhart or Gangrel ever defecting and joining Chrom's army, so it's safe to assume this didn't happen in her timeline and there's no way for them to father any of the gen 2 kids.
While I'm not completely banning S supports between them and the mothers, I will keep them restricted until a reasonable explanation for the kid's existence within what we know of Lucina's timeline is provided.