How Do I Check An Xbox Adaptive Controller Setup Is Actually Mapping Correctly?

Yes, this works, and it is one of the better uses for a tester like this, because the Adaptive Controller's strength is also what makes faults hard to trace: it collapses several physical devices into one gamepad, so when something does not land you cannot see which layer dropped it.

Here is the useful part. The Adaptive Controller presents to Windows as a single standard Xbox gamepad, and the tester shows you the output of that whole assembly. So if you press your external switch and the corresponding button lights up in the tester, then the switch, the cable, the port and the Adaptive Controller's mapping are all working, and the fault is in the game. If it does not light up, the fault is somewhere in that chain, and you can then isolate it by moving one thing at a time. That single distinction removes most of the guessing.

The method that works best is to test one input at a time and write down which index lights up. Build yourself a short reference list: switch in port 1 lights index 0, joystick moves the left stick axes, and so on. Keep that list. When an input stops landing later, you can check it against the reference in thirty seconds instead of swapping hardware.

For intermittent faults specifically, which is what you have, press the suspect switch slowly twenty times while watching the tester. Count the misses. An input that misses in the tester is a hardware or mapping problem. An input that never misses in the tester but still misses in a game is the game's input handling, and that is worth reporting to the developer rather than troubleshooting at your end. Also check your mapping in the Xbox Accessories app, since a profile can be assigned per-game and switch underneath you without any visible sign.