Since you are trying to contact Travlerajm without success, I'll try to answer some questions
Does air pressure/temp have an impact on mgr/i?
I don't think so.
Arm length can vary among people of the same height. Some people have more fast twitch muscle fibers then others. Also muscle mass affects mgr/i. Bearing all of this in mind, shouldn't everybody find their own mgr/i?
It's about the natural pendulum frequency of the arm. I think the weight and lenght of the arm are the factors that determine that frequency. So the mass influences the Mgr/I (that's why the wristband theory comes in play)
According to Travlerajm, you apply force in your forehand stroke when your arm is almost at the lowest point. The musclefibres influences the accelleration in a forward direction. At the beginning of the forehand where you allow gravity the let the racket drop, your supposed to be relaxed. So I don't think they play a part in Mgri/I.
Using a smaller grip size is said to give you more wrist snap/range of motion in your stroke, allowing more spin on serves and groundstrokes. I'm sure you're aware of this trend. However, mgr/i seems to be about using a relaxed wrist. What is your opinion on grip size?
Mgr/I is indeed about using a relaxed wrist. Players who use a whippier stroke tend to have lower Mgr/I and thinner grips. With thicker grips it's more difficult to whip upwards.
I found it is best to tune on court with a playing partner who doesn't mind interruptions.