Good question. Definitely, there will be a soul gem issue. I don't think there's any way around that. My understanding is that you could go about two days' journey away from the turtle before really feeling the effects. (This may need to be double checked with the mods; I thought I remembered it from a reference to how far Shay's ship could travel away from the turtle.)

I was thinking potentially that those who are scouting could travel along with Shay and use the ship as a point to return to when they get tired or start feeling the effects. They will have a limited range of what they can see in front of them, but that's still advanced notice -- and if my understanding of range is right, it should be pretty decent advanced notice.

This kind of range is also why the scouts would probably need to be pretty fast/create some kind of communication chain in order to get information back to the rear of the turtle where the movers are.

Does that help?