I myself am expecting these OCs to be villains and to get some sort of comeuppance during the story, which would not be very Mary-Sue-ish at all. Of course, that's just a gut feeling of mine. My bigger worry is that having OCs means they'll need to flesh out the Lunarian society in general, not just the Lunar War, which could greatly expand the scope of the story to something so big that it'd require an anime of its own (as opposed to just a 20+ minute episode or two).
Just like their character designs being biting off more than they can chew with animating it, they might be biting off more than they can chew with the scope of the story now that the scope's being expanded to include Lunarian politics.
I don't think ZUN's ever outright said it, but the hints are obvious enough. She has those Omoikane spellcards, Yagokoro-Omoikane-no-mikoto is one of Omoikane's names, and ZUN calls her one of "the highest, noblest gods" in the SoPM interview.
Also, maybe relevant, but as stated in Silent Sinner in Blue, "Eirin" isn't her real first name (but Yagokoro is still her real surname).
I thought the reason youkai didn't succeed was unrelated to Eirin's trap. I am pretty sure it was a failure from the beginning, unrelated to the trap.
The thing that the trap did was prevent at least one youkai from escaping punishment, which was Yukari.
Eirin's trap was set after the first Lunar War in case Yukari ever came back. This is stated in Silent Sinner in Blue, IIRC. The way the trap was set required it being set centuries in advance (by altering the rotation of the moon slightly so that centuries later, the way to get into it was offset by a day which would totally screw with whoever was trying to enter, that way. This is stated in both CiLR and SSiB, IIRC)
General indications are the youkai simply got their butts kicked. Yorihime just refers to Yukari as "troublesome" while reminiscing about the war to Reimu in CiLR, Akyuu said they had no chance against the Moon's weaponry, and Yukari tells Ran in CiLR that they can never win because the Lunarians possess such technology and "unyielding stamina and abilities that far outmatch youkai" (possibly a piece of evidence that Yukari started the first Lunar War knowing they'd lose).
The poster might be confused because there is evidence that the trap was used twice. In addition to the present day time that Yukari tripped it at the end of SSiB, Yukari implies Ran in CiLR that it hit another time as well when an "anonymous youkai" (that was probably Yukari herself) came to the moon just to look around and play. Far as I can tell, given that Yukari tells Ran this story before the conclusion of SSiB, yet Yorohime seems to think that SSiB is the first time the trap is sprung, an explanation for Yukari's story (if her story actually happened, though the narrative implies it did thanks to Yukari's suspiciously specific denial about it) would require a time paradox or time-travelling shenanigans. (or possibly Yukari sharing Maribel's memories in some sort of parallel-ity shenanigans thing)
Granted, my memory is a bit hazy (it's been years...)
