Excellent points. I should keep in mind your point about not using the third reef as a substitute for a trysail. Having said that, the weight of my new mainsail will be the same as the weight that my sail maker proposes for the trysail. I assume the trysail has stronger stitching and reinforcing, but maybe this will be built into the third reef. If the stitching is strong, that final reef will be very sturdy. The sail area will be low (about 60'), so there will be much stress on everything than from a larger area of sail -- but of course in very high winds even that tiny bit of sail will be subject to tremendous forces. I like the boat to be at ease as much as possible (I make changes anytime heel goes over 15 degrees), so I expect to use that third reef quite often in winds far below the point where a trysail would be necessary.
Regarding your comments on trim, you remind me that I must check the headstay tension (mental note: add this to the to-do list). I doubt I'll ever have perfectly trimmed sails. I do try, but there are so many variables that I usually settle for "good enough," especially when not going very far. My first mate thinks I'm anal about sail trim, but I think I'm lazy about it...
I sewed the trysail from a Sailrite kit. It is all triple stitched, 10 oz cloth, with very heavy patching and heavy hand-sewn corner grommets. They are obviously made to take a beating that would shred a regular sail. The shape of a Trysail seems to be different as well as it gets the maximum wind force lower than a third reef would be. I would love to have a second mast track which many boats have for the storm sail but will have to tie up the main tightly and drop the boom to use the trysail. I don't see a real reason to remove a mainsail completely if it gets wrapped tightly around the boom. Maybe AFTER all else is accomplished it could come off. I also bought a sea anchor and made a drogue which hopefully all stay buried deep inside a locker along with the trysail but it's nice to know they are there.
I have never deployed these pieces of gear in heavy wind which bothers me a bit. I made up some well though out, large print, detailed, step by step laminated instruction sheets which are right in the bags so the procedures for deploying (especially the sea anchor) can be reviewed before making some big mistake and putting the cart before the horse, like getting a line on the wrong side of the lifelines, etc.
After dealing with excessive weather helm, I shortened my headstay by close to 2" when installing new rigging. It made a big difference. That is definitely something to check, especially on a boat with a large mainsail like the older designs.