I know, I know, I know you shouldn't split the party. But I wonder if it might not make sense in this case.
Suppose they left Reynir and Lalli with the cattank. Reynir stays safe inside. Lalli can go out and get water, fuel, maybe food; as well as detect trolls. Meanwhile Sigrid, Mikkel, and Emil take Kitty plus what weapons and survival gear they can carry, using backpacks etc -- forget about the wheelbarrow, it would just slow them down -- and head for the pickup point. When the pickup people show, a larger crew goes back to get Reynir, Lalli, and the books -- and maybe the rescue ship carries some kind of vehicle to do so with.
The major problem I see is that the people heading for the pickup point would have no mage with them. But they'd have a troll detector in Kitty; and the ghosts, at least according to Reynir, seem to be fixed on Reynir and might well not go beyond the cattank once they catch up to it -- and the cattank is protected against them.
The other problem, of course, is how to explain this to Lalli -- 'no don't try to follow us, stay with the tank and protect Reynir, we'll be back'. But Mikkel could explain it to Reynir, at least. Though it would be unnerving, waiting in the tank for people to come back for you who might not have survived to do so.