Questions about 12v socket panel

Shouldn't matter. Of course, you should use your multimeter to be sure you have 12v to that fuse location.

I would recommend a minimum of
12l AWG wire for pretty much any 12V circuit. That is to reduce resistive voltage drop in the wires, which is much more of an issue at 12V than at 120V. 12AWG can handle 20A so a 20A fuse will protect that circuit and hopefully keep you above the 25% rule Colt is raising (load = 80% of fuse is the same as fuse = 125% of load). . And yes the currents for all the loads you have on that circuit are additive.

The fuse protects the wiring and sockets from fire. Select your fuse by the size of the wire, not the load. Also, do not load a fuse over 80% of its rating. That means an add-a-fuse on the wrong circuit could overload it. Look to see if you have an extra fuse place, or 2. My 180 has 1 extra 12V space.

That would work quite nicely. It also gives you a much better read-out for battery state of charge than the panel LED lights. I'm guessing that the switch will turn off the unit and stop any unnecessary power drains. If you are considering putting it in the bed area, consider one without the meter and install a separate meter. The light from it might be annoying at night.