Hello TopCat:
I can certainly understand the constraints, either real or "vibed" that might be implied by the other "officer" on your boat. For me, that's one reason why I have bought flea-market priced used, or repaired, or re-cut my sails over the years instead of getting new. (Also because all I want is a sail that will do reasonably well in my usual sailing conditions. And say $100 in DIY work for maybe 90% OK performance -- Or $2000 for a new sail for 100% performance that will deteriorate to 90-95% in a few seasons anyway?)
Looking at your picture P1210584.jpg:
1) Yes, the lower-most grommet on the luff will serve as the cunningham. Normally one would run a line up, then through grommet, then down through a block(s) so that the lower section of sail luff can be pulled down and tensioned. For your purpose, maybe just running/tying a permanent line from the grommet then around the boom to shorten the luff a few inches will enable you to tension the whole sail luff before the head bumps up against the block at the top of the mast. Warning though: When hoisting the sail with the cunningham pre-rigged, just put firm tension on the luff with the winch. Don't over-tighten or it is possible to tear the sail at the cunningham grommet.
2) I notice that the boom goose-neck is affixed to the mast via a large bolt which goes through the mast. Can you instead drill a new hole for the bolt several inches (you determine how much) further down and re-attach the boom at the lower level? This would cause the boom to be more horizontal rather than drooping. And instead of (or also with) the cunningham, the lower goose-neck level would enable you to tension the luff before the sail's head bumps up against the mast-top block. (Note: Give thought whether this suggestion might compromise the mast strength. Personally, I don't think so as people drill holes in masts and booms all the time. But ascertain this on your own or with opinions of others.)
3) As for the boom end being too low, this is certainly because the sail is too long on the leach -- compounded by the fact you can't raise the mainsail's luff fully. Example: On my boat, I purposefully have a sail with the leach probably being a bit too short in relation to the luff. So my boom hangs above horizontal a bit. Being 6'3" I need this to make sure that the boom height is always above my head when I am standing in the cockpit. Keep this dynamic in mind if sailing with the pop-top up will really be required. The leach needs to be short in relation to the luff. Or, you need to get a smaller sail (luff length) and then raise the goose neck location on the mast. (Just the opposite of my item 2 description.)
Looking at your photo of the whole sail up. My observation is that the sail fabric itself is reasonably tensioned in relation to the luff rope. In other words, a shrunken luff rope isn't the primary cause of your issues. Yes, it is possible that the luff rope has shrunken, but it's mainly that the sail is quite a few inches too big on the luff for the boat.
My observations might not be spot on --- but maybe they will provoke insights for you to further evaluate alternatives.
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