If [English], a splendid hentai doujinshi parodying the Fate/Zero series, by Yukimi

… The premise you described doesn’t make sense. Kayneth is an egotistical asshole whose overriding priority in life is his status as a magus from a distinguished bloodline, spits on the very existence of “heretics” like Kiritsugu Emiya who treat magecraft as a mere tool rather than the lofty and hallowed tradition that traditional magi like Kayneth hold it as, openly and publicly humiliates one of his students for suggesting that hard work could help a magus compensate for a lack of a centuries-old legacy, and when he finds himself unable to have things go his way he’s more than willing to break the rules of the Holy Grail War by backstabbing the officiating arbitrator just to monopolize the advantage of having acquired extra Command Spells (a very big deal in the Holy Grail War).

Sola-Ui, meanwhile, turns out to be little better, as it turns out that she WILLINGLY gave in to the power of the mystical curse that causes women to become infatuated with Zero!Lancer just by looking upon his face (unless they have a degree of magical resistance — which Sola-Ui has), gives zero fucks about Lancer’s insistence that he has no interest in reciprocating her affections, and has zero compunctions about torturing her already crippled fiance (the aforementioned Kayneth) in order to finally acquire the Command Seals that would give her true authority over Lancer. Oh, and she’s more incompetent as a Master than even WAVER VELVET, and unlike him she refuses to accept this as an argument against her directly involving herself in the Holy Grail War unless she risks giving her beloved Lancer a bad impression (and that goes completely out the window once she gets her Command Seals).

In short, Kayneth and Sola-Ui helping Diarmuid and Artoria in their romance? Nonsensical. You’d have a much better argument with Kiritsugu and Irisviel doing so instead, since not only their own relationship is much more healthy and stable, but they’re much more competent in terms of fighting the Holy Grail War, and at least Irisviel is a believable candidate for supporting the two Servants’ romance AND having both the metaphorical balls and ability to walk up to Kiritsugu “Kill Hundreds to Save Thousands” Emiya and dare to argue for helping Diarmuid break free from his master rather than, say, kill him in a gruesome and horrible fashion using a really underhanded trick.