It could be used with SS, silver solder, brazing, or TIG welding are better choices with TIG being the gold standard
Per Oatey: How do I flush flux out of my water lines? You will start off by getting a five gallon bucket and small electric pump with two washing machine hoses. Mix 1-one pound box of TSP (Tri-Sodium Phosphate), 16 ounces (2 cups) of bleach and hot water in the five gallon bucket. Drain enough water from your system to allow this mixture to be pumped into the water lines, and circulate throughout the system for one hour. The best location for hose connections is the hot and cold washing machine valves. Drain the system completely and repeat this process again. After the second application, you will want to flush the system out through each fixture for at least 15 minutes.
I think yes, if an air feed line is a copper metal. You can test it with a copper wires, how bond is strong.
No, this will not work with aluminum.
Yes you can.
I've been an electronic design engineer since the 1980's. The universal rule in the electronic industry is to never use plumbing flux on electronic or electrical circuits. The main reason for this is corrosion. Even if you are extremely careful in removing acid plumbing flux, you can never be sure all of it is gone, particularly from any crevices where corrosion would like to start. The other reason is the flux may be conductive, or absorb moisture from the air over time to be conductive. If it's a high-impedance analog circuit, slightly conductive flux can cause it to not operate properly. This is why most electronic fluxes are rosin based. Rosin flux is not corrosive at room temperature. Most rosin flux is still removed anyway with an appropriate solvent during manufacturing. Note that flux core solder used for electronics comes in several flux grades, from pure rosin to rosin that has been "activated" for better cleaning action. One commenter recommended watching YouTube for advice. Keep in mind you don't need any credentials to make a YouTube video. YT is great for metal and woodworking tutorials, but some of the soldering videos have misinformation. Actually, I'm just a design engineer, for a proper answer to this question, you need a team with a material scientist, chemist, mechanical engineer, etc. If you run a manufacturing plant you have those people or your solder supplier does. That said, if this only needs to work for a few months and you're able to clean off the flux without damaging the components, then good luck. It will make nice joints on copper pipe.
The flux is for plumbing soldering. It is not for electronics, rosin core soldering. Once the copper pipes are soldered, they will certainly conduct electricity.
Probably, depends what metals you're joining