I feel like you're on the right track, but we need to take it a step further. I say we grab both of them an clonk their heads together and then shout something like. . .
"You! (points at Red Knight) You are going to explain exactly why you are so angry at Kathrine in exact clear detail!
And you! (points at White Knight) You are going to stand there and take it without interuption until she has said her piece.
And then we're going to talk this out like reasonable mature human beings, or I swear to Gaia I will kick both your butts until you decide to see reason!"
. . . or something like that.
A little aggressive, but probably on the right track.
I actually like
@kelllogo's plan there. It could use one or two elements from the other one though.
As for the people who just want to stand between the two girls with Spells that can probably rip Solid Core apart in an instant... bad idea is what I think.
So let me take some sensible parts and put them together...
[X] Plan (Un-)Reasonable People
-[X] Transform and point out to both of them that Magical Girls like them fighting in a town is a bad idea; can't they at least take it somewhere further outside?
--[X] Whether or not they do it, tell Red that you would like to hear the whole story from her; make sure White does not interfere, it is obvious her old friend does not want to hear it right now
--[X] Try to work it out with words if you can; make an attempt to calm both girls as best as possible
---[X] If fighting breaks out, protect White from Red's regular attacks and protect Red from White's regular attacks; tell them neither of them is reasonable right now, so you will beat them both if you have to. Mention that after Goddess Grey and White Soul, this one does not scare you much
----[X] Go about beating them with intercepting their attacks, not actively attacking
----[X] In case either is insane enough to use her Spell, try your best to dodge it with your fast speed
[X] EXP-Plan Damage Increase
-[X] Purifier Beam to 20 (570 EXP)
So... a general idea what to do, a direction to go. No direct speech because SV shoots itself in the foot far too often with that. Leave the wording to Crys.
Additionally, a brief plan in case a fight starts out. Nepgear is the mediator here, so she must not fight just one of the two; even if one is her friend, she needs to stay as neutral as possible if she wants to get any credibility with Red.
I am reasonably certain that the only things they can actually hurt Nepgear with are their respective Spells; those will kill her if they hit, so dodging is the only way.
Also a few levels for the Purifier Beam; the previous argument was that Apoc would sprawl out of control... but now it does not.
This brings Base Damage up to 22k/44k, enough to beat the Knights, but not unreasonably high.