If you are just running the C12 jumper the gauge wire will not matter as it is only a data line. I don’t use anything bigger than 0.02 I know the smaller gauge is harder to work with but you want the chip sitting as flush as possible. On this repair it doesn’t matter if it is insulated or not. I prefer insulated.
As for using soldering mask just make sure it doesn’t impact the chip area or the chip will not sit flush.
If the C12 pad has come off don’t just use a straight wire, make a pad with the wire by forming a circle.
It is a good idea to re-ball the IC with leaded solder and use a pre-heater when putting the IC on to avoid heating the baseband CPU to much.