Two points, first we need an alternative servant to Hercules then
Yeah, I don't think Heracles would be a good idea as a servant allied to the heroes. He's just too damn powerful, particularly if Ilya was smart enough not to summon him insane. Plus, like I said, it doesn't make logical sense for her to summon him (or, at least, it's not a certainty), given that it was the Einsberns who found the catalyst.
point two kiritsugu post fourth is a nice guy, at worst he'd incapacitate masters
Even someone who is Kotomine's "daughter"? Although, admittedly, her being friendly with Shirou would help her case somewhat....