What is the difference between the reverse and the regular mount? Which one has the motor closer to the center of the skateboard?

Normal/Inward Motor Mount is facing towards the center of the deck. Reverse/Outward Motor Mount is facing away from the center of the deck.

What size are the threads on the two different threaded holes?

We use an M3 Thread for the Belt Cover. M5 Thread for the Belt Tensioners/Idlers.