So in conclusion, if flash is mounted, there is no way to automatically get standard exposure in Av mode.
Also it is undesireable to use Av mode, simply because there is no way I can make my Min shutter speed to 1/90 or 1/125. Also Auto ISO is Clipped to 400.
My intention is, I want the camera to automatically get exposure of ambient light 1 stop below standard exposure exposure (given min shutter speed and auto ISO), in different lighting condition. And be able adjust FEC as necessary.
As conclusion There is No Way to do this.
I often use Av mode for flash stuff for candids, etc, because it does meter for the ambient. I'll typically set the meter to -1 and then adjust FEC accordingly. Set the ISO manually high enough so that the ambient is about 1/100-1/200. Enable HSS on the flash in case you walk into a brighter area and the shutter speed goes past the sync speed of the camera.
It might be worth checking out the manual to see if you can override the max iso of 400 constraint. Is it only with flash attached, etc? Perhaps there is a setting to remove it.