To visualize, when setting the Envelope lower than the cutoff value in the filter does this essentially flip the diagram you included in lets say the first video of lesson 9? So that the envelope sets your darkest point and cutoff sets your highest but playing from highest to lowest instead the opposite at lowest to highest and back (with release/sustain)?
Yes. The attack stage would take the cutoff down to a darker point determined by the negative filter envelope amount, and the release would bring it back up to the cutoff. The sustain level would range from the cutoff knob setting (when set to 0), to the darkest point (when set to Max) as determined by the negative filter envelope amount.