Sora won't align with darkness, him and Ven are RoL, maybe he succumbs or sacrifices somehow unto the darkness.
I don't think the X blade is abandoned, as previously mentioned, it was MX's scheme after all. I highly think that it may serve purpose again, especially in reference to use either on Sora/or+Ven in the RoS. I think Xehanort's plan is to get Sora in the RoS, make him confused or mind hustle him into a trap of sorts.
I think now that the Xehanort seeds are in 1Vanitas 2Braig 3Isa. They all had undergone either a receiving of, or sharing of some darkness that is somehow of Xehanort's signature in appearances and abilities. Unless there was someone who caught the XS(Xehanort seed) prior to Isa that we do not know of, it would be in that above order. Xehanort's whole ploy with his heartless+nobody were apart of his plan, but not "seeds" in the way that Van, Braig, and Isa are.