Not much help, but I can tell you that on my Universal M-18, if the raw water pump
shaft is not all the way in the slot on the shaft that drives it, there is no way the cover would go on over the impeller, so it sounds like you are good there. Next thing I would try is priming it, though mine has never needed that and the thru-hull is well below the raw water pump
. Third thing is confirm that the hose after the raw water
pump is clear and not kinked or blocked and finally, I have heard of some impellers that have separated from their hubs and the impeller will turn with no load, but will not pump when the housing is sealed up. Hang in there, its probably some small thing causing the problem and driving you nuts. Oh, the key is in the keyway, right?