Funny, if your goal is making 100% Windows compatible OS, then WTF your OS needs to be "Unix-like"? Like why? This is something from "we don't seek for easy ways"?)))
I am working now on my own OS exactly on this frigging ARM, both 64 and 32 bit. But 1) the goal of being a 100% compatible OS with such a complex program set as Windows is, is UNFEASIBLE. ReactOS sucks on this for more than 2 decades now. and 2) I am not a Unix fan. Like at all.
But I wish you success! It's so good to see someone took a look at something else other than x86. It's so so so boring to see all these "triple fault GDT, CS, protected mode, VGA not working qemu...". 
Good luck!In fact, in some far future, my OS is gonna be opposite to what you wanna do. It's NT-like but having POSIX-kind-of-compliant environment subsystem. This choice was motivated because I love NT and because there is a lot of available POSIX implementations, so that my OS, if succeeds, wouldn't be left without programs to run.
What the boards you've chosen as targets? Don't only tell that RPi!