Well, I guess we used the command spell. Probably for the best though. I've spent so long playing F/GO that I've forgotten how bad Madness Enhancement can affect a servant. As it stands, it seems to me that Mordred at this point is like Lancelot in Zero, completely incoherent and can only be pointed in a general direction, or Spartacus in Apocrypha whose is as liable to kill their own master as they are to kill anyone else in the grail war. Unless Mordred's Spirit Origin can be modified, there is no chance in hell that her and Shirou can understand each other, or helpful in any way other than as an unstable bomb.
[X] [Illya]
My reasoning for this choice is that I think Shirou needs to develop a bigger rapport with Illya. He spent a small amount of time with her, but that time was well-spent bonding, or at least getting on her good side. Morally, I think it would be good for Shirou to grow closer to Illya, give her some happiness in her life since it's up in the air at the moment if she is going to live past the next decade (a lot can happen in a grail war, so her ultimate fate is dubious right now). Practically, Rider (or Medusa, if we don't care about using aliases) is a great servant to team up with due to her mobility and ability at close-ranged combat, which both Medea and Circe lack. Rider's master being Illya shows that Rider is not impeded compared to when she is paired with Shinji, and Illya is knowledgeable about the servants, the grail war, the 3 families that created it, the holy grail, etc. Things which Shirou definitely needs, even if some of this info is no longer relevant due to whatever chaos is happening in the war right now.