I'll chime in on a couple of your questions. This is how I did all of our in-wall wiring/cabling for our room...
#1. I have a 40' run of Monoprice HDMI cable and found they had the best price and their quality is as good as anybody else's and no, you will not have any issues with that length as long as you keep a foot and a half or so of parallel runs from electrical wiring and make a 90 degree cross whenever you need to cross or pass electrical wiring. (This goes for all A/V wiring and cabling).
#2. I used all 12 ga throughout our room, that would be my suggestion but 14 will be fine.
#3. I bought a spool of Tributaries RG6 and made all of my subwoofer runs with that and terminated the ends with wall plates and RCA plugs ... again, long runs will not affect SQ.
And #9. I found the wall plates to be much easier to use and makes for a much cleaner installation, just 'plug and play' and yes the speaker wire can be connected to the backside of the plate and banana plugs can be used on the front side. Some installers like to hang the bare speaker wire out of the plate for a straight connection to the speaker but I used the banana plug wall plates.
