BRG worked
To be honest, I'm not sure why BRG worked when Plat guaranteed availability was not offered when I referenced the Expedia rate, but somewhat happy with the outcome - 10% off expedia rate at some calculation I'm not sure I understand.
I'm still cool with SPG - just wish things could be more uniform/simpler at times (i.e., don't make me BRG a room when it is available elsewhere)