Charrua, no appologies necessary when good questions are asked !
1) As long as there is no danger to the pool, nor the security of the cover, the water is customarily lowered to below the returns by many people. In some areas with a high water table, pools can suffer from having water removed from them (as you've safely lowered yours below the returns before, I'd assume that it's ok to do it as part of your closing)
2) You can do that at any point after you're done using the 'backwash' or 'filter' function of your multiport. I usually wait until I'm done blowing out the lines and setting the multiport to 'winterize', but you can do as you wish (unless the pressure gauge doubles as a drain port for the multi, in which case you want to leave it in to ensure that you get enough pressure to blow the MD) (BTW - some multiports do have a separate drain plug [9/16" wrench removes it] that should be removed if the unit is going to stay outside)
3) The purpose of the second plug is to keep you from losing the water in your pool. As a fine point, the purpose of the second 'port' is to allow you to connect either the main drain through the skimmer line ( !!!!!!!!), a 'bounce/ ballance' line (an open pipe into the bottom of the skimmer that will "supposedly" keep the skimmer from drawing air if the water level gets too low vs. the skimmer opening) or a reserve line that is plumbed in at the time of construction to allow a 'quick and easy' skimmer line replacement in the event that the main line breaks (it's plumbed up near the main, ie working, line, but not plumbed into the pump unless the primary line fails). I'll assume that the pool builder didn't use the port for any of these, so glued the plug in, often from the bottom.
4) http://www.poolforum.com/pf2/showthread.php?t=5973 - this gives my take on antifreeze and amount needed.
Again, welcome to the forum - I look forward to seeing your 'answering posts' next year, as you learn more from the forum and about your pool, and as always, feel free to call upon the great(!) folks here to help you with any questions you may have - Waste