I will answer each question in order.
Yes, I show a 3A fuse in line from the DPDT switch to the positive bus because that is likely 16 gauge wire and we don't want a high current spike coming to our fingertips. Note that the fuse should be placed as close to the bus/source as possible, not up at the switch, although you wouldn't know that from the drawing. Fuses are actually protecting the wire as the higher gauge is not meant to carry high current.
For the accessory bus, note that I did not actually do this setup in my rig. I just did the altered wiring diagram for you. (Happy to do it.) I may add the DPDT in the future, but currently do not need to have accessories on without the key on. In any case, you do not need an actual bus bar for this - you can just tie the 3 leads together. I would probably solder them and put a wire nut on. The thin high gauge wire does not stay together that great with just a wire nut. I wouldn't trust it on it's own. Or maybe solder and electrical tape. Or wire nut and electrical tape.
To wire the key-on power to the DPDT, you just take the lead from the key-on source directly to the switch. The video goes over several methods. I bought the key-on harness from the SxS club, but because I am an idiot I did not figure out how to actually reach the back of the cigarette lighter until I was shown how stupid I was and that I could have easily reached it once the switch plate hole was cut out. I ended up using a fuse tap, which I actually like as it was very simple and less than $7. See the video for details on that. Instead of running the key-on lead to the stinger, you are going to run it to the DPDT switch, and run the stinger to the accessory bus (or your wire nut trio).
My setup has the key-on power source going to the stinger, but yours is not.
