As much as the 'boundary' idea is nice, I'm not sure it works like that. Put it this way, it drained us enough unsealing Byakuren from a boundary. How are we going to fare if we try and place one on someone else? I'm certainly not leaving that down to chance.
But the idea of using our gap is not out of the question.
If Momiji is aiming for Hatate, which I think (and for this plan, I'm counting on it) she is, then if we simply side-step, she'll think we're letting her have at her.
That's the plan, however.
Momiji is quicker then us; so simply reaching out and grabbing her will be useless. With our gap, we can stick a hand out at a point where we know we can get her, and do this:
Grab the back of her collar and pull her back, and down, in one swift movement with that youkai power we're so known to have. At her speed, it would give us the result of Momiji's legs leaving the ground, her body falling backwards at an alarming speed, and the back of her head cracking the floor.
The result should give us a very dazed, if not, unconscious Momiji. If she isn't KOd, the grip on her sword would be loose; so we should be able to kick it away.
At that point, we have Nitori restrain her... if she has anything on hand. If not, we restrain her ourselves; or with the help of those in front of us.
If she is KOd, we simply get her inside. I'm not leaving her; but I will be happy if she is restrained so she can at least listen to us if she tries to strike out again. Maybe get Hatate to talk to her and try to fix things between them. (Yes, I'm playing on the Hatate story again; but of course I am, she is my favourite tengu!)
As for any other plans, throwing danmaku out might not go down so well; though we're probably more prepared this time since we can see her coming. What Hatate does, I don't know; she ran away from Momiji in her Quest, so I'm hoping she might help. I can't see her trying to stop things by simply talking to her; but I really don't want a piece of Momiji whilst she has that sword in hand.
The other idea is to gap the sword... but that is easier said then done; and I'm not gapping her arm off. That's just mean and uncalled for.
Another idea could be gapping her right into the wall itself; but I'm not sure if that would have the desired effect, not compared to the added whiplash effect of the original plan.
Damn Dormio for not being around for once. >>