Re: ION and HDMI/Optical Audio
To the best of my knowledge, there's no way to work around this while still using the UseEDID option.
Can't you set the custom resolution without specifying UseEDID? Perhaps your custom mode fails validation based on EDID constraints such as H/Vsync frequency? There are some other options to turn off that validation, without preventing the EDID from being read. See the driver README.