[Propshaft]

Ok. The roof is all one piece with those roof channels just part of the panel. So no gap for the water to leak into the cab.

It does look like somebody has put a sponge seal along the channel, probably to cover the rust.

So I would just leave it alone for now and when the weather gets better pull out the rubber seal, rub it down and paint it up.

Sorry, thats not 100% correct. underneath the significant bead of seam sealer, there's a spot welded join. see the bottom of page 3 here, you'll see how its made up: Al's D40 project | Page 3 | Nissan-Navara.net

As part of a respray, i dug out the seam sealer and redone, also windscreen out to get to all the rust and blast/treat fully. I would say unless you remove the seam sealer completely and the front windscreen, you'll struggle to fully address. (arguably you'll never fully cure rust on spot welded joins)