I've been using the Marquis for a bit now and love it.  I also own the Voxformer.

Can you explain to me how the "key filter" section works?  I've reviewed the description and I just don't get it.  It sounds like the lower the one sets one of the curves, the stronger the compression.  can you also describe how to use the Program Dependent Release Contour?

Also, and I feel really stupid asking you this question, if, for example, I want the compressor to begin kicking-in at -15 dB, I would set the threshold knob to -15, correct?

Finally, what does the green bar at the bottom of the middle part represent?  Is that the overall reduction of dBb's?

Key filter filter outs specified frequencies so that compressor affects them stronger or smoother.

For program dependency, I suggest you to check out this forum - it's a tough topic to describe in a couple of words - unfortunately if you are not a DSP engineer it can be a mysterious thing.

Threshold should be set depending on the knee setting.  If your knee setting is 0dB then compressor will be affecting signals above -15dBFS, otherwise it will start at lower levels.

Green bar in the middle bottom is an achieved gain reduction.