Glad you like it!
Don't try that, you will kill your config. MP is not (yet?) compatible with PX4.
Why do you arm if you don't want to spin the props? You can always use a kill switch if you prefer direct throttle control.
If you disable the GPS check circuit breaker under Circuit breakers you won't be able to arm without GPS.
That's available. Configure the tuning channel in the tuning view and then select any param in the param editor and map it to one of your three tuning channels. This setting is not persistent across power cycles for safety reasons.