My thoughts:

Source G'raha Tia (aka past-self G'raha) is going to play a part in getting the Scions back to the Source. How? Not sure, but I think we will have to awaken/interact with him (perhaps that's why the Crystal Tower raid is a requirement to go through 5.3 and going forward for new players). Perhaps we have to return to the Crystal Tower on the Source and somehow interact with Source G'raha Tia to successfully return the Scion's souls to their bodies.

I do think First G'raha (aka future G'raha that transported the Crystal Tower to the First) will eventually disappear in some fashion due to the timeline change. Once the Scions return to the Source, that version of G'raha Tia won't need to exist anymore; he won't need to go back into the past to stop the Rejoining. He's fulfilled his mission by saving the two worlds and getting the Scions back home. Once this is complete, I think that's then he's going to disappear in some way or another.

Essentially, we won't really loose him. Just that version of him. We still have Source G'raha Tia. They are one in the same person. The First G'raha is the future self of Source G'raha. They aren't like the WoL and Ardbert; they aren't a soul split due to the sundering.

Just my 2 cents.