Electrical – Help creating a half hot switch: 3 sets of wires

Based on your readings, the 1 side black, 2 side white(! retag this wire with black electrical tape!) and 3 side black should be nutted together and pigtailed, and the 1 side white should be pigtailed to the 3 side white apparently -- there looks to be some sort of load plugged into the outlets connected to the 3 wires, which'd explain why the 3 side white reads 120VAC with regard to the 1 side white. From there, you can then connect the white pigtail to the silver screw, the black pigtail to the lower brass screw, the 2 side black to the upper brass screw, and the bare pigtail to the ground screw.

The funny voltage readings on 2B with the switch off are due to the high impedance of your meter seeing capacitive coupling of a small amount of energy into the switched-off wiring.

All you need is a short length of black 14AWG and a nut

The always-hot is already present in your receptacle box at the nutted-together black wires with the one white wire coming in. So, once the power's off, take a short piece of black 14AWG solid wire (THHN works), nut it onto the existing junction (you'll probably want to use a new nut for this), and land the other terminal on the spare brass screw on the receptacle after breaking off the hot-side tab. You should also take some black electrical tape and flag the white wire that runs from that junction to the switch at both ends with it while you're here, by the way. Button it back up, turn the power on, and enjoy your half-switched outlet! (What you're doing is "pigtailing" the receptacle screw to the existing junction.)