Hi LG! Welcome to the forum and congrats on your "new" DaySailer. Answering questions in order:
1) Yup, I have the same "three point" tabernacle on my mast. Not to worry, it's plenty strong. Besides the mast is held up by the stays, the tabernacle is more to keep things centered and in the right place. That being said:
a) Those stays are crucial, get them replaced (drmarine.com, $180-ish) if you have any doubt about them. Also very good to have the newer 1/8" stays.
b) Be careful not to let the mast twist off to port or star as you are raising/lowering it, you'll bend the plate. Don't ask me how I know.... a little is fine, but don't drop the mast at a major angle...
c) When you raise/lower the mast, make sure you have both side stays attached properly - running over the spreaders and attached to the chainplates. Again, don't ask me how I know...
d) I often forget to put the pin thru the little hole in the forward vertical pin/post. It's not really needed, but nice security, I guess - yes, when I dropped the mast, the little pin sheered immediately. Anyway, use a straight cotter pin, not a split ring, less work.
e) This tabernacle has a little play in it. My mast will "hinge" off to the leeward a few degrees, maybe translating to a foot at the masthead. I've learned to ignore it, was scary at first. Actually, most of the play is between mast and tabernacle (the socket), not the hinge plate itself. Anyway, don't sweat it, get good stays.
f) I do have a "normal", "four-point" tabernacle sitting in my parts drawer. Doesn't fit my mast without major machine-shop reworking of the socket. I figured that new stays were a better investment. Actually, at that point, I'd prefer a one piece mast.. one day.
g) The pin that holds the aft hinge together often jams when I'm putting it in/taking it out. Solution - my 2lb deadblow mallet and (as needed) a cheap phillips screwdriver to use as a tap/punch. Before the mallet, I just banged on it with an old wrench, shame on me! The pin itself is a long clevis pin, I replaced mine with stainless steel just for ease of mind.
2) You'll be fine, the mast jack pretty much gets jammed into the keelson by the pressure of the stays. I'd add a larger diameter washer to spread the load. Then just make sure it's tight enough so that you can't move the jack around (fore/aft) by hand. I think you're saying that the jack also pivots fore/aft on it's bolt? Again, just tighten the bolt so it's not all sloppy and let the stays do their work.
3) My guess is that those blocks were put there by previous owner to hold up the carlins, and by extension the side decks. Translation, carlins are ribs that hold up the side decks, kind of like rafters under a roof. Check your carlins (look up the term here in the forum), any dry rot? cracking? broken? missing? I'd get rid of those blocks and fix the actual supports in the near future. But as long as the deck is solid you may be able to get by for now. Anyway, this warrants some close inspection.. stick you head under there, stick your phone under there and take some pics, feel the give on the side decks, look fro surface cracks, poke the carlins with a screwdriver, etc.
4) Beer holders! Nah, my guess is probably turning blocks for the spinnaker. I'd fill the holes whenever you can get around to it and have some spare thickened epoxy or the like.
Oh my, I almost forgot. Do not store a DaySailer on a mooring! Maybe a few days, but you're asking to lose your mast. Even moreso with a tabernacle of any sort. If you must, take the mast down.
All the best and upgrade your stays! Here's the link at D&R:
http://www.drmarine.com/proddetail.asp?prod=DS800Tom