I'm no physicist either, just a dumb golfpro.
Having looked up a few of the terms, I think you are very probably right about the weight changing rather than the mass changing. I was refering to the idea that a weight on the end of a long stick moving fast is "heavier" than a stationary weight on the end of a short stick. Maybe this is "inertial mass" - I don't know and probably never will, but it doesn't change the aforementioned concept.
I don't think anyone really knows enough about physics to represent the golf swing completely correctly. Its just such a big mix of many forces interacting with each other. I'm not even sure whether any law of physics can be viewed completely independently anyway. I'm clearly at my limit in terms of the knowledge of physics required. That's why Brian has teamed up with those guys, cause there aint many physicist golfpros or golfpro physicis about on our big blue ball.
)But as I say, I'm just a dumb golfpro who is trying to understand as much as I can about the swing. BTW, the more I understand the better I hit it!
Regarding whether it has an effect on the flat left wrist, the amount of Negative Left Wrist Torque would have to increase if the effective weight/inertial mass of the clubhead increased in order to maintain the FLW, no?