Thank you pdqaltair and paulk.
All excellent observations and I'm still working on this.
I have removed the snap shackle between the jib halyard and the upper part of the furler. (Hated wasting that lovely splice, but it was either that or cut the shackle.)
The spinnaker halyard block will not move off to the side. The dip in the "crane" holds it extremely solidly in the middle. I've taken it, from deck level, all over the deck and it won't budge. I went up the mast and tried to move it by hand, and it won't budge. Removing all tension from the spinnaker halyard would work, but that can't be done from deck level, because simply pulling on it to try to move it applies sufficient tension to lock the block solidly in the middle of the dip in the crane.
When I say "solidly" I mean it won't slide sideways one iota. When the mast is down and there is no halyard on it, then it is completely free.
The "crane" is welded on to the mast-head fitting. I could consider having this re-machined over the Winter. Clearly "something" should be done. Perhaps I can clamp some small fitting on the crane which would hold the block off to one side.
In the meantime, I have also looped both "arms" of the spinnaker halyard around the back of a spreader, and that does seem to keep them off the forestay - the one arm just barely off, but not touching is not touching. I'll keep an eye on the halyard for wear as it wraps around shrouds, and of course will have to free it up when I use it.
Related dilemma: the forestay has shortened this year
. So far, both my #2 and #3 foresails are too long on the luff, with a little sag left when the upper furling mechanism is fully hoisted (and it shouldn't be fully hoisted anyway). I may have had a little of this last year with one of the sails last year, although ...
... in order to shorten the exposed halyard and prevent halyard-wrap when furling, I added a 6" wire "pennant" to the luff, which worked like a charm. This year it's too long, and I have had to remove the pennant. Now, I did add one more shackle into the mix, at the tack, a snap shackle (I know!) for easier sail-changing. (I'm thinking of not furling my new headsail, but taking it up and down for every weekly race.) But that's nowhere near 6". Nothing has changed to my knowledge in how the boat is rigged, and this is my 4th Summer of ownership.
How could such a thing happen, that the forestay this year is a few inches shorter than last year, or conversely that last year was anomalous and it was longer that year than usual? Just weird. Apart from the backstay turnbuckles, there is nothing to adjust without disassembling the furling basket etc., and I haven't done that. It's a deck-stepped mast with no adjustments a the step itself.
Also, I have a D-shaped shackle at the tack, which fits the lower part of the furling mechanism, but it requires tools to unscrew and screw in the pin (to make it firm enough for my satisfaction), which is not impossible but seems laborious on occasions with few crew and a pitching deck. There was a quick-release type of shackle, of the general type pictured below, there at one point, but it also required tools - perhaps it was simply damaged and I need to replace it. Thoughts?
Thank you all again.