Cannot Progress From "The Fallen"

[PennyS]

Going to make this quick:
I haven't joined the leigon or the stormcloaks until now(because I didn't realize it was important), and because of this, Tallius wants to give me the "JOIN THE LEIGON B CUZ WE R KOOL" dialouge instead of the "Lets go postpone this war so we can stahp a dragon" dialouge. I tried to keep going back to my last save and talking to him at different times but he won't give me the right lines. I tried joining the leigon to see if he would give me the right choice after but now he's just giving me the lines for the leigon quest chain. Is there a way to force a quest to think it was completed so I may progress?

This is less of a big deal: Can I change my faction at this point in the game? I did not realize at the beginning that going with Ulfric or the other guy would determine your faction and I would like to join the Stormcloaks.

(I'm still new to this game so anything stupid I say should not be held against me in the future)