i think i have an answer... dont shoot me if it doesn't work for you.
i was using win7 32 iso with a sandisk 32gb flash drive.
i also got stuck at the at the country selection - with no mouse or keyboard working.
i was using a tool called NTlite, that can inject drivers into an iso.
i tried just about every USB3 driver set i could find, none worked.
i started to suspect NTlite was not injecting the drivers.
so i started to google how to inject the drivers manully.
i came across this website.
i followed the pretty simple step by step guide...
Bingo.. it now boots win7 32 with the keyboard and mouse.
http://nucblog.net/2015/07/installing-windows-7-on-the-nuc5cpyh-or-nuc5ppyh/
hope this helps,
Gilly