[major]

Hi jehan,
The angle of brush centerline to commutator radius is irrelevant to direction of rotation. It's primary purpose is to reduce chirping or bouncing. That 22.5° angle is used for decades on all Prestolite motors which rotate both ways.
Of more concern is the factory brush advance of 4.5° favoring its 'as shipped' rotation. That can be changed by redrilling the holes in the aluminum comm end head casting. The relief pockets for the 4 screw heads are wide enough to accommodate the additional hole. After doing that, the motor will work perfectly fine.
major

 

[major]

Really none. It's doubtful that you'll see much over 100V at the motor even with higher V battery. RPM would be excessive. If commutation is too sparky, look into split wafer brush.
major

 

[major]

I don't think it's terribly critical to +/-1/2° . You seem concerned so I see nothing wrong about doing 5 or 6°. Watch for interference with connectors when twisting the casting. I just don't see need to drill and tap new holes in steel frame. Slots? Some motors are designed with that type of adjustments. I feel it's unnecessary and even problematic. We're talking a few degrees which are within tolerance of brush seating so can change as brushes wear.
major