I can answer some, but not all, of your questions. A 2-speed winch is
pretty common, and in fact is preferred to 1-speed. There's a lot of
sail to hoist, and most of that is pretty easy work -- perfect for the
fast speed on the winch. But you'll want to get the halyard tight at
the end of the hoisting, and that's work for the slow speed. Actually,
we use 3 steps in hoisting the mainsail on our 50-foot cutter: First,
we simply haul on the halyard. This gets the sail to about the upper
spreader, when its weight more or less balances one of us. Then, the
high speed for the last 10 feet or so, and finally the slow speed to
put some tension on the halyard. Note that the halyard will 'creep',
i.e., stretch a few inches and then stop stretching. This is due to
straightening of the fibers in the halyard, not to stretch og the
fibers themselves, which is miniscule.

I assume you mean the exit blocks low on the mast. You might be able
to get away with no blocks at all, as we do. But be very careful to
check the lead through the exit slot to be sure there's no chafe. I'm
pretty sure you can still locate exit blocks with double blocks, though
I'm not sure where.

Boom reefing. Have you looked at cheek blocks with brackets that
run fore-and-aft? That solves the spar curvature problem, but might
not work in your case. Other than that, lots of pople use pieces of
wood, cut to fit, or filled epoxy molded in place (with wax applied
liberally in advnce, so the block isn't permanently attached to the
boom or to the cheek block).

No idea about your SS screws.

Of course you can fill unused holes with filled epoxy -- or just leave
them; that's what we do.

Rob