A lot to unpack here. Starting with the easy one, I don't really have any idea what the tab is for on that swing arm. The QS motors come with a bracket that attaches to the motor shaft for mounting the disc caliper. But the one depicted is in an odd place for caliper mounting.
The switch you've depicted is actually pretty massive in real life. I recommend a kill switch and
a battery disconnect (for maintenance) switch. Since real estate inside the frame is at a premium, I use this marine circuit breaker as a disconnect. I think Jim uses the same one too. Small, light, and effective.
As for brakes, EmVeeTee installed a hydraulic foot brake, which is an option I probably would use if I built my scooter now. I think SIP sells them and it is awesome. I like the handlebar brake, but the hydraulics crowd an already small handlebar and you have to find a way for the hose to travel through the frame on their way to the back of the bike. If you do use the foot brake option, there is nothing for your left hand to do, since there's no more shifter, so you have to get past that.
As for the drum vs disc argument, I agree that you can stop a stock vespa with a drum brake, but you can stop faster with a disc. The hub motor is a giant electromagnet, heavy and a large spinning mass to bring under control. I would argue a disc is the better way to exert that control, but I also dislike telling another brother how to build a bike.
Dual shocks is indeed a better ride, but heavier than the mono shock design. If you can live with that, go for two. I think JimVonBaden and EmVeeTee would probably tell you the real problem in swing arm design is that the disc brake and the shock want to occupy the same space, and that results in some compromise on design. The drum brake is one compromise, while the shock and disc brake on opposite sides is another compromise. If you do go for dual shocks, I think you are locked into a drum brake, as there is no more room for a disc. Just food for thought.
It looks like you have some fabrication to do with your floorboard. There are a lot of members with some real skills in metal work on this site, so your progress will likely be followed closely. It might be slightly diluted since you're going electric, but welcome to the EV club.
This is a 72V 200 amp switch.