Based on what I'm getting at here and how the current pages are I'm gonna go out on a limb here and say it's either Remilia taking the win or a stalemate at best.
The main thing Xeno a has going for them here are the Passive Probability Manipulation, the Durability Negation and the Power Null and while I do think this is a bit of a complicated debate, I don't really think this beats Remilia.
To compare the main 'probability manipulation' of both, Xeno a's ability is indeed passive but seems to lack complete control over and/or has only a limited ability to manually control. Remilia's ability, while not as over the top counts as both Fate Manipulation and Probability Manipulation, and considering this scan here it's implied that it is both passive and also something she can manually bring out without much effort as well. So in this case, Xeno a isn't the only one with a passive probability ability, albeit Remilia's isn't as over the top it should be able to at least defend to a certain degree against Xeno a's.
So, Xeno a might have basically a passive probability-based invulnerability around them that will be able to defend them, there is nothing stopping Remilia from being able to hinder or defend said probability hax via her own to at least a certain degree, if not completely cancel it out. This would also by extent mean the option of Xeno a to limit Remilia's movement would also be something she'd defend against since that ability is also based on probability which would also be something that could be partially hindered or cancelled by Remilia's own Fate Manip. And even if she doesn't activate the manual use right away, the mere fact hers is also implied to be passive to an extent should mean she should have at least a small bit of counter-defense at the beginning. There's also the whole Acausality Type 4 thing being a defense against Probability Manip that could easily be argued here iirc but that would only make this be a stomp in Remilia's favor.
Going on to the other things Xeno a has; the Durability Negation or Power Null. Considering the attack potency of Remilia's abilities and her durability, it would all be at around a
2-C level and Xeno a doesn't have anything (at least on the profile and as far as I'm aware) suggesting they're even capable of dura negging or nulling something on that level since speed is the only thing equalized here. There's also nothing I've seen of so far that would suggest this being able to get around Remilia's regeneration which works on a Mid Godly scale. If that's the case then probability-bypass or not Remilia would just be an immovable stone wall that Xeno a has no hope in actually finishing off either.
So this leaves us with two scenarios: One where Remilia either shrugs off or partially counters Xeno a's probability hax then wins via AP stomp, or stalemate where neither can actually kill the other because one is invincible due to passive-probability and the other has impassable regeneration.
On a bit of a different note but still on topic, and not sure if this is actually a valid outcome but based on the fight conditions I got the mental image of the fight starting, Xeno a's passive prob hax kicking in, Remilia's counterhax then kicking in, Xeno a making one move then Remilia sniping Xeno a from the other side of the battlefield due funny range difference.
