Is it safe to lets say, shave 2-5 MM off the hub of the wheel if there is sufficient amount of metal on the wheel?

 

Yes u can, but the wheel studs are gonna stick out extra mm when u put the wheel on. Not sure if ur gonna need open end lugs.

 

It will depend on the wheel.
I wouldn't go milling material off without an engineering analysis.
I'm available, btw, M.S. Aerospace Engineering, I've designed/analyzed wheels on road-going production vehicles

 

You will lighten the load capacity of the wheel. If they are an OE wheel they are designed with enough load for the car they are going on. If you were to mill material off the wheel they are going to be under loaded for the car.
Just be careful.