I'm expecting that the Scions (plus Ryne) will be able to return (or visit, in Ryne's case) to the Source by the end of the MSQ, because that would be a possible explanation for why MSQ progression is required for the last fight of Eden's Verse. Ryne, Thancred, and Urianger are heavily involved in that, so having them acknowledge their new ability to go back to the Source is reasonable, as opposed to having a mini-time bubble just for that segment.

Having said that, I do wonder if this ability to go back to the Source is only temporary. That bit in the trailer with the Scions Plus Ryne popping into existence to protect Cid is a little odd, if the Scions were already physically present on the Source.