Graham, your problems make me scratch my head: My experience with cambered HM sails is good, and I have also found that the taller their AR is, the less attention is needed to adjust the running parrels. Johanna’s sail with an AR=1.87 is close to the lowest I would have (but I may get away with lower AR if I don’t cut the masts so short). Your medium-high AR sail should be easy to make set well.
I could think of twoo reasons for your problems:
Your mast is too short for your sail so that you were forced to cut the yard short. This results in that you get problems with the force of the halyard working too far forward in the sail. A throat hauling parrel may help against this. On my Johanna the mast is also on the short side so the slingpoint on the yard has been moved as forward as I can without getting handling problems (aft-heavy sail). With a foot taller mast I could have moved the slingpoint a bit aft. This would make the sail set with less attention to the LHP and YHP.
Another reason could well be the forward rake of your mast. When you lower the sail, naturally the halyard will always want the sail to fall forward and thus create negative batten stagger. The fact that forward rake works well on the lo-AR fanned sail of Fantail, may not be a proof that forward rake is also good on a medium-high AR Hasler-McLeod sail.
What to do? Lengthening the mast is probably too much trouble. I would re-make the top panel with a full length yard. Then I would experiment with the position of the slingpont, starting on the mid-point and even try it a little aft of that. This will not let you hoist the whole lowest panel any longer so you may end up sailing with 6 panels until you can re-cut panel 7 to about half size.
The rake of your mast is probably also fixed so you have to live with that and use the throat hauling parrel to control the batten stagger as you lower the sail. Wasn’t that the way you do it today?
If building a HM-style sloop JR from scratch, I would recommend using a perfectly vertical mast. On a schooner I would have a vertical main mast and maybe 2-5° forward rake on the foremast. This ensures a better barrier against accidental gybes (caused by turbulence from the mainsail) and also moves the CE forward.
Arne