You’re going to get buried responding to this original H90 thread.
I believe you answered yourself in 1. It’s “almost like working with a real tape machine”, which I see as the intentional design with HeadSpace.
It’s not easy to search / hunt down links prior to the format change here. But I did find some informative ones from Eventide that should help with 2. & 3.
https://www.eventideaudio.com/forums/topic/modfactor-fx-types-definitions/#post-117130
https://www.eventideaudio.com/forums/topic/mix-control-vs-intensity/#post-117434
https://www.eventideaudio.com/forums/topic/anyone-else-screaming-for-a-mix-on-modfactor/#post-136629
