There’s a lot to unpack here.  Not a staff member, but maybe I can help give Eventide a little head start.  I’ve done enough forum research on these same topics to link a few of the results below.

1. ADSR will work with guitar, sometimes better due to the sharp attack.  It has a fixed shape of short durations for each stage.  It does need fine-tuning of Sensitivity for the best results.  Try it on some type of deep effect to get a feel for it.
2. IMO, the 5th: TriceraChorus.  But also see the descriptions in the ‘link-of-links’ below.
3. The Instant rack FX do have unique Mix ranges, but also see discussions in the ‘link-of-links’ below.
4. Specific answers found in the ‘link-of-links’ below.

H90 Questions ‘Link-Of-Links’

The ‘other’ 4). and 5). – These almost need their own dedicated reply, because you’re right: there are many combinations to cover.  I’ll try to expand with more specifics later on.

• A good place to start is to set Depth & Depth Mod to median values.  If Speed is set fairly low, then dial the Mod Rate multiplier higher.  Speed fast, Mod Rate low.  It’ll be easier to hear the effects of each on the modulation, and tweak it from there.
• It also helps to start out with a predictable Shape & Mod Source (Sine, Triangle).  One method I use is to map the [P] HotKnob to Shape, then the [A] HotKnob to Mod Source.  Makes short work of trying out all of the possible combinations.
• Always map the Manual parameter to an EXP pedal (if possible), whether it’s in current use or not.  That way, it’s always active when Manual is selected for Shape and/or Mod Source.  In some effects, it’s better to ‘reverse-map’ the Manual parameter from 100-0.