Can PWM safely be considered as effectively lowering voltage?
PWM does not lower the (peak) voltage. PWM reduces the average current (and in consequence the average power).

In case of LEDs - they don't care much about the voltage, it is the current that can destroy them (both forward and reverse). So you have to take care not to exceed their peak current rating.

To reduce voltage you would have to add an inductor, catch diode and capacitors to make a buck converter which does lower the voltage.