On a good day 100W flat panel in Ca will harvest 15-20 AH a day in January and 30-40 AH in June. You won't get enough on some days in winter, and battery will slowly go lower but it will take maybe a whole bad week to drop to 50%, with your 15 AH consumption.
Though I don't understand how you can live on 15 AH a day. Fridge control circuit alone draws at least 10 AH per 24 hours. Maybe with no furnace and very little use of LED lights and radio, you can.
1) Check
List of completed projects.
I know that Search doesn't work well, so briefly what was recommended by many and what I did too, for anchoring and sealing:
3x3" or 4x4" aluminum angle about 6" long, with 1/4" pins or machine bolts into the side of the panel (with rivet nuts if you use bolts), and #10 anchor screws into the roof plywood (2 per bracket is enough in your case), with plenty of Dicor under the bracket and over the screw heads. I also lubricated screws with Dicor before driving them in.
There is already a few dozen screws in your roof not counting those holding plywood to rafters, and another 8 won't change anything.
Needless to remind to sailor, but for other people reading - use stainless hardware only. Manufacturers are being cheap and use regular steel, but there is no reason to repeat their mistakes.
2) Nobody runs cable inside the wall. What people often do, is drilling the hole in the fridge metal vent side and routing the cable through the wall of the fridge cabinet and then through the top or bottom cabinets. Yes, plastic channel on exposed areas.
3) Yes, battery cable through the floor or through the front diamond plate using cable strain relief.
4) If spare and main battery is same specs and same number, i.e. 2 main and 2 spare, you just charge them all. I don't see a need to discharge one deeply while keeping a spare one full and then swapping them. Maybe I'm missing something here.
