Low Level Input/Output on subwoofer

As long as you use the low pass crossover built into the plate amp for the subwoofer that should work fine. The sub will then only play the lower frequencies that you dictate and the low level output should be transmitted to your active monitors unmolested.

Yes, of course. But you would want to know the exact nature of what is passed via the outputs. Is it a full-range signal or is it high-passed. Either way, you would adjust the sub's variable low-pass accordingly.