First off I do not have any use for some one "making up" B.S.
yeah! we'll tell you what we know and make up what we don't know.
Jon is just joking. You would do well to pay attention to anything he has to say. Few on SBO have more experience then he does.
"The notch seen in the third pic, i know is of course used to put the boom, and sail on the mast. When you need to lower the sail, how do you prevent the plugs from falling out of the grove?"
For a trailersailer, like the 21 America, the designers expect the sails are most often removed when lowered, but you can buy a track throat plug that will fully capture the sail slugs that ride in it.
"When the sail is up and you are "sailing", is the glide at the end of the boom suppose to be secured to the cleat near the bottom of the mast, or do you let the boom vang, keep the boom in place?"
There is a piece of tackle called a Cunningham that can be used to pull down on the gooseneck (the glide/swivel at the forward end of the boom). The purpose is to allow tensioning the sail's luff for fine tuning. A vang is for more control over the over all flattening of the the sail, but the sheet is the primary control that also keeps the gooseneck from lifting. I don't know how the 21 America is rigged, but some boats position the boom below the track throat while a few hoist it above the throat. There is a whole book's worth of knowledge just in how to use these trim controls: vang, sheet, traveler, Cunningham, down haul, halyard, and out haul. Even the way the sail battens are tied in and the topping lift can contribute to special needs in sail trim. Not all sailboats have all of these pieces of equipment. The sheet, down haul, and out haul, are on almost all sailboats.
"The main sail has the gromets for two positions of "short sail". I have a plan on how I will set up the needed rigging at the aft end of the boom, but what do I do at the mast end to secure the gromet?"
These are your
reefing points. There may be a reef hook to use on the forward gromets.
-Will