There ar 4 sheaves on the front of the mast at the masthead. The two inner sheaves are for jib halyards, the two outer sheaves are for spinnaker halyards. I have jib halyards on the inner sheaves (primary port, secondary starboard) but do not use the spinnaker sheaves as there seems to be too much chafe on the halyards where they exit the mast. I have instead a spinnaker crane with swivel blocks with the halyards exiting the mast about two feet below the masthead on either side and going up and through the swivels which eliminates most of the chafe at that point. One has to be careful where one secures the unused jib-halyards however as they will chafe the spinnaker halyard between the exit slots and the masthead swivels if one is not careful about securing them clear.
There are also 4 sheaves on the back side of the mast at the masthead, two for main and spare halyards and two more for lifts although I only have a single lift rigged. I do, however have a messenger rigged for the other sheave that can be used to haul a replacement halyard, or topping lift, up and over the spare sheave should that be necessary. (All of my halyards are 10mm T-900 which is a little slick but pretty bullet proof.)
The two stacked sheaves below the steaming/deck light are actually both for topping lifts, one each port and starboard. Because of the baby-stay, the boats were designed to be raced with two spinnaker poles rather than just one. When one wants to gybe, the lee pole is raised, clipped to the lazy sheet, the opposing after guy taken up and the opposing working sheet tripped. Bingo--the spinnaker is gybed and the lazy pole lowered to the deck and secured. Not too many boats were delivered with two poles however (mine was not) and so one must use a dip-pole gybe with the added difficulty of freeing the pole from the mast fitting, passing it forward and around the Baby-stay, and reconnecting it to the mast. That takes a couple of coordinated guys at the mast and leaves the spinnaker "flying" for a few minutes and in any serious wind--say 20+ knots or more, can be pretty exciting. If you're really going to race, buy yourself another pole and mast fitting (available through Rig-Rite) and use the two pole gybe. It's safer and easier. (Or don't and just cruise very quickly!)
A few people have attempted to use the stacked sheaves as you describe, hauling a wire stay to the mast with the upper sheave and using the topping lift in the lower sheave as a halyard for a staysail or storm jib that's hanked on, but the sheaves really aren't sturdy enough for the loads that may be generated in serious conditions, at least in my opinion.
The 4:1 Purchase on the running backs was the factory set-up with wire rope runners. The wire rope is tough to deal with and, if the boom rubs against it anywhere it will eat up the boom. The Spectra runners eliminate that issue, weigh less and have less windage, and since you're not using the secondaries when you're using the runners, can be more than adequately set up with the secondaries. Remember you never really see more than about 2000# on the runners which is easy for the Spectra and secondaries. (You only want to set the runners up enough to keep the mast from pumping and in column.)
Yep. I still have the 4-108. It is a great, sturdy, engine and with up-keep will last a long time. I have about 3300 hours on mine and, with a 2-blade Gori folding prop, have average fuel consumption of slightly under 3/4 Gal per hour at 2500 RPM while making 7+ knots (albeit with a clean bottom).
"It is not so much for its beauty that the sea makes a claim upon men's hearts, as for that subtle something, that quality of air, that emanation from the waves, that so wonderfully renews a weary spirit."