[X] You'll talk to Archer's faction while Emiya finds Assassin. He might have a better chance of convincing Olga if he finds her. Archer was going to be tricky given the whole cult thing but you'll have to shoot your shot.
[X] You'll talk to Assassin while Emiya meets with Archer's people. It's going to suck but you'll have to do it. It's your mess after all and you're not going to sleep right until you made things clear.
[X] You'll talk to them both and have Emiya as your back up. Safety in numbers but this is going to take a longer time and the ball may roll while you're putting off one for the other. Still, it's more secure this way with two Servants and Masters.