As you know Al, I have always hated Mendelian genetics as a way to explain what we see in discus. It doesn't not allow for co-dominance and penetrance, among other things. Discus certainly are not binary. Things really get wonky in domestics when you start dealing with multiple mutations and their interaction.
I am going to have to think a bit on your question about the lethal homozygous gene theory. I just know it being recessive doesn't fit.