Hi I'm a newb with the construction set, I was interested with the game settings changeable in the contruction that alteres min and max walking, speed, time you could breath underwater...etc.

I was wondering if you could change it during the game with scripts. To the effect of something like:

set fMinWalkSpeed to 150

as a line of code.

Nope.
I'm pretty sure that even MWSE (the script extender) can't alter GMSTs in-game.