Can I use the PWM pins as regular digital input/output pins?
Yes you can use them as ordinary digital io pins. PWM is essentially a normal digital output pin that is repeatedly turned on and off at a high rate. 

The magic is that the turning off and on is done in the hardware according to the duty cycle  (I.e. there is no need for software to perform the on/off operations) . This is why only some pins can do h/w PWM, because the PWM hardware is not added on to all pins.

If you are not using the pin for PWM, it is available for you to use as required.