Considering a custom rifle for the GF that is in a McMillian stock, I need to trim the LOP at least an 1".

My question is, how much can you trim off of an already delivered McMillian stock?

Better check with McM first, will probably void warranty.

No biggie to shorten a LOP on a McM and install/grind a new pad. Done right it won't void the warranty.

There's a few tricks to it, at least the way I do it btw. PM me if you need help.

Well, it doesn't matter, I missed out on the deal researching this, wanted the rifle for the GF who is 5' nothing

Fwiw, I used to order McMillans without recoil pads at full length LOP, then trim the buttstocks and install the pads myself. A bit of work but they came out slick.

McM produce a youth stock?

Not that I know of but a cut down compact would be close.

According to one theory she would take a 12" LOP but that sounds way short, I would try some different stocks if you have access to different LOP stocks. I would also consider a Bell and Carlson which I would have no concern over cutting down or modifying. They have a youth stock with a 12.5" LOP that could be a drop in solution if you are going Remington.