zonon The air unit has no mounting holes. I have seen 3 ways of mounting: Pull tied to the frame like Flywoo (there 3D antenna mounts are the best), Doubled sided tape like the Beetle, and various 3D printed mounts. The air unit seems to be able to take a lot of punishment inside the frame which I can vouch for personally. The biggest detail about mounting is making sure the antennas can be secured safely and so there is access to the SD card and USB slots. The antennas don't necessarily need to be sticking straight up like you can see on many newer PNP quads that come with the air unit installed.

zonon Yes but you have to activate it in Betaflight

zonon The flight controller regulates the power so no, you don't need anything else to run a 6s.

zonon It depends on the actual size of the battery since it has to fit in the battery compartment.

zonon My board has no RX1 pad so I used RX6 and activated serial port 6 in Betaflight

zonon The 1507 motor will not fit the original CX3 frame, the hole pattern is different.

zonon It sounds like your receiver might be getting hot, causing it to have a weak signal or shut down completely and then the failsafe activates. Check out the solder joints and plug wires and try moving it or try another one and see if it happens again. The antenna could be damaged, so check it out and the solder joint there too..