but right now I'm stuck on how to efficiently use a pwm constant-current sink LED driver to switch a pnp transistor to switch an N-Channel mosfet to turn the lights on and off real fast.
The constant current limit bit goes right out of the window. Basically you need a pull up resistor to 5V from the output. Then you connect the base of the PNP through a resistor. You seem to have done this but I would change the resistor values and have 10K as the pull up and 1K as the base resistor. Then that collector signal should be fed into the gate of the FET. Again you seem to have done that so I can only suggest that you have not got it wired the way your schematic is.