How would he get Diarmuid? Did he steal the catalyst from El-Melloi (which made him furious, considering he had been stolen from twice
, and summoned a Berserker instead?
)? If it's just a "standard" summoning with Kariya having the same motivation as canon (save Sakura),
then I can't imagine that the Grail would send him Diarmuid. Granted, I'm not too familiar with Diarmuid's backstory, but I think his thing is more with love entanglements and treachery, whereas Kariya's thing is more of the standard "rescue the maiden from the evil wizard". Maybe he could get Don Quixote XD.
EDIT: Scratch the above. I could see how that would work, actually (Aoi and Tokiomi and Kariya).
As for survivability, you're asking us to consider this hypothetical scenario of a Servant swap in the 4th HGW we're familiar with, though at the same time you admit that the conditions prior to your version of the 4th are very different. Unless you are willing to tell us what exactly has changed, I'm afraid that the best we can do is give you predictions based on established canon.
Worm'd Kariya + Lancer = dead. Magus Kariya + Lancer = Kiri's gonna be gunning for him, so also dead. Magus Kariya + Berserksalot = Possibly dead, though has a better shot than most - though as Cherry pointed out, Zouken wouldn't allow that to happen without worming Kariya first.