What you have mentioned about the bolt only turning slightly each way. We had similar once in our workshop, bolt came undone ok but after a few turns the bolt got tighter & tighter until it was seized solid, most odd. We called in a local chap that specialises in removing broken bolts etc. He drilled it out, the thread on the bolt had picked up the thread and had locked solid, did not damage the thread in the crank as the crank is so hard.
It could be similar in your case.

Alan

The problem has been fixed with a new crankshaft pulley and key being fitted.
Cost just over 400 bucks but I consider that cheap compared to what it would have cost with a new crankshaft as proposed by the unmentionable repairer in the northern suburbs WA.

It took the repairers a long time to remove the old one and they don't recommend the bolt has lashings of Loctite when being fitted, as it's a fine thread and when torqued up correctly, will not come undone.
The key had hammered the loose pulley keyway and the noise emanated from this as a "wow wow"or ringing noise.

I put the theory of a water crossing with a hot engine possibly causing it to slacken off but this was thought to be highly unlikely.
Just thought I'd let you know for those times when funny hard to trace noises start, as it gives something else to check as a possible cause.
Alan.