The only five years later problem is solved thusly: My Akasha-obsessed mage cabal has a few members tapping into the leylines to force mana to accumulate exponentially faster. They actually end up with too much and fourteen masters get chosen in order to burn off the excess in the form of extra Servants. It was either that, or everyone gathering mana explodes. I suppose they could also somehow rape a bunch of people without removing their hand from their conduit to the leyline they're working on, and diffuse the mana through their victims, but... No, extra servants it is.
Okay, so Medusa's out of the equation, I guess, but what if El-Melloi II used the cape that Rider gave him as a catalyst instead of the scrap of cloth he stole from his greasy blonde predecessor? Might that give him a Rider that had all his memories up until the cape was bequeathed to his Master?
Diarmuid I might be able to work around also by having the curse he invoked upon his death affecting him.
I think that's a different Holy Grail in Grand Order though, isn't it? Takes place in the Moon Cell or something? It might work differently there.