Thank you all for your thoughts on this. Fist of all, Eddie (Sailermaned) was right - once the tension was off all the various lines & halyards, determined use of a screwdriver under the head of the pin got it to move! There is a clever little spring inside the axle pin that prevents it from shaking out - plus there was a certain amount of salt buildup that was keeping it from moving. But mystery solved! Thank you again.
The stuck halyard itself is a different problem. It seems to me the line is the original boom topping lift - it's the lightest of all the lines in the mast, 3/8" or perhaps less. The problem started so long ago, nobody, including the owner, can remember what was what or what happened. But what happened to me yesterday was that the halyard finally parted somewhere in the middle of the mast, leaving the messenger stuck inside somehow. I had removed that weird hose-clamp thingy from the end of the messenger and spliced an extension on, in case i could get the halyard moving. Then I started hauling on the two ends, harder and harder until - bang! - the line broke and it came spilling out of the masthead - and then it got stuck again. :-( Now there's nothing for it but to send some pretty 70-lb deck wench up the pole to look at the masthead for me (I don't think the winch is made that can haul my 200-plus pounds up there!). Then later, a plumber's drain camera maybe, to find out what the messenger is stuck on. <sigh>