There are reasons to try testing in the order of Stabilize, Alt. Hold, and finally Loiter. Further, I have always setup the Plane functionality first. Once the plane is properly calibrated and tested in Manual and FBWA modes, then it is ready to continue conversion as a QuadPlane. In fact, this process has made my two conversions quite easy using v3.6.0 firmware.
In the initial QuadPlane Q_Stabilize mode testing, this is where you would find channel reversals. I don't recall having any issues nor do I see and special Q parameters to reverse direction for just the QuadPlane function. It is important to set the Q_FRAME_CLASS and Q_FRAME_TYPE correctly and follow the QuadPlane Frame setup wiring diagram.
Good luck and Happy Holidays!