I don't know if handling 8-ch audio correctly for DVD output is worth all the trouble...
My workaround is to preprocess source clips with 8-ch audio using dmMediaConverter. This is an FFMpeg GUI specializing in video conversion, and it is very smart. Source clips with 8-ch audio are shown stream based, you can easily select or discard specific audio streams. If you want to downmix audio streams you can either do it into WAV or into the desired DVD target audio format. For video you just use "Copy", and the target container should be MKV. This MKV can be used as the A2D source file, and I never had any issues with the conversion.
Cheers
manolito