Hm, well, let's break it down.
Kariya and Tokiomi: HAAHAHAHAHAHAHAHAHAAAHAHAHHAH no.
It's actually not quite
as implausible as you make out, at least in this scenario. The two of them are not actually that far apart in terms of their goals. They both want Aoi and her kids to be happy. They just have very different philosophies and are both terrible
at communicating with each other.
In canon, those things make such an alliance impossible, but in an AU where Kariya has learnt magic properly it's actually possible that he'll understand Tokiomi a bit better. Further, in such an AU Tokiomi will respect him a lot more, since he hates Kariya because of his attitude to magecraft. And, if they can get past their fundamental hatred of each other they don't actually make bad allies since, as I said, they both want the best for Aoi, Rin and Sakura.
Kariya and Kiritsugu: Kerry's always been more the lone wolf type. He'd only ally out of convenience, and would probably end Kariya as soon as he ceased to be useful... much to the dismay of Arturia and Diarmuid.
Kariya and Kirei: Hmm. That... might actually happen, considering how Kirei was so treacherous in the canon 4th. Diarmuid wouldn't like it, though, since his Master is insinuating himself back into the "love" triangle, and Diarmuid knows how those tend to work out for him and those around him.
Kirei would likely stab him in the back, though, so that's probably a no go as well.
Yeah, I can see an alliance forming, but I do not
see it ending well for Kariya.
Kariya and Waver: I don't know what you're planning for the story, but an inept magus of a prominent bloodline allied with the magus who believes that magi can become great through knowledge rather than birth would be interesting. There wouldn't be too much clash with Iskander and Diarmuid as well.
Yeah, actually, I can definitely see this pairing. Their goals don't clash, as such, and even if they did end up fighting over the Grail I see Waver as being reluctant to kill anyone
, let alone a former ally.
Kariya and Uryuu: Hoo boy, that would be interesting. Could be a case of using monsters to fight monsters (Kariya tries to manipulate Uryuu by "promising" to teach him how to do Magecraft?). Not sure how wise it is to ally with serial killers, though.
Yeah, I can't see them working at all
. Kariya isn't going to stand for gratuitous child-murdering, particularly in the city where Rin and Sakura live, and Uryuu is just too insane to ally with anyone
. Plus, it would basically guarentee
Kariya's death, not least because he would be a clear villain who wouldn't deserve
to live if he remained allied to Uryuu even after seeing what he did to those children.
From a pure survivability standpoint, it actually looks like Waver is his best bet, since he's mostly reasonable, not insane/ruthless/treacherous/a complete asshole, and actually survives the War in canon.
Yeah, I'd probably agree. The fact that it's an AU does change things a bit, though, particularly with regards to Tokiomi.