A lot of suggestions. Get the evidence, is the shaft all one piece to the prop? Is the coupling flange you twiddled tight to the shaft? Do you have a rope cutter, one of those guillotine ones, not a circular one? If your prop had been loose I would not have expected it to stay on or drive for long, If it was loose it would have been driving on the key alone and and I would have expected it to have sheared by now.Engine is very poorly and out of the boat.
Cleaning out the empty space today I casually twirled the coupling. Seems like there is a couple of degrees play in both directions before the prop turns.
Is the prop loose on the shaft, and if so why? Don't want to add a Cascais liftout to the cost of fixing the engine. (Plan is to have a wee sail around the Algarve then lift out at Bruce's if there is space, so will fix wonky props etc then, but will it fall off before then?)