Paul, I share your trepidation. Would have loved to have seen a video of John Allen constructing his G & D Squaw Valley mountain range.
Your effort far exceeds what I have done so far. Just ignore the critics, many of them have nothing. A genuine craftsman is usually rather gracious.
Like your self, I have shaped some 2" foam multi layered basic shape small mountains.
However, for my large range I will be using expanded metal stucco lath. That medium sure is great at drawing blood!
I have built up some long 12" embankments by hammering the lath over small chunks of lumber cutoffs which will be covered with casting gypsum fabric.
For the much taller mountain range we will be taking full sheets of lath down to the new rip rap rocks surrounding a new interstate I-95 bridge abutment and form a mountain shell over the large rocks with a 15 pound hammer. The larger rocks may yield more dramatic Yosemite level of shapes.
Do not know what I am doing but that never stopped me.
Hope to pull it off in the early morning hours.
