We do not have information about the BO Pass. Please find out here if it is valid on the Glacier Express. Seats on the Glacier Express can be reserved here.
In order to have the same seat reserved for yout trip from Brig to Milano, please make one single reservation/ticket for the whole leg. You can enter the Half Fare Travelcard reduction so that you can benefit form your Swiss Half Fare Card all the way to Milano. Please book your ticket here.
I wsh you an excellent Sunday.
Kind regards, RahelW