One way of adding a bit of redundancy to propulsion would be to use both a set of conjoiners and reversers which would function with a bit of delay. That way, the pair of the chulls would be moving one ship in the same direction while moving opposite directions themselves. This would also allow for one crew to keep pulling the ship while the other changes directions.
I think that Roshar at the moment is not advanced enough technologically to create an entire structure out of aluminum. I suspect that they don't have a way to even get enough metal. And IIRC there was a WOB saying that it would be hard to soulcast aluminum, but I may be wrong about that.
As before, I'm not sure if this technology even exists on Roshar. This leads me to believe that methods of local propulsion for those types of ships of sufficient power and efficiency would be magical in nature.
Another idea I had was to use a pair of platforms floating on water, in a cistern of sorts, and adjust the vertical position of the main platform by changing the water level in the cistern. While the speed of climbing and descending would decrease, the safety of the crew of the ship would rise as the failure of the mechanism would result in a gradual descent, rather than a sudden crash to the ground. This would also allow for the ship to remain afloat at least at some distance for the ground if the water was drained from the sides of the cistern and not from the bottom, leaving the ship out of reach of regular foes. But this method is also flawed and I struggle to say if it would be better than the current one.