Finally, I found that when I use an analog connection (a vga cable with adapters) the custom mode lines are used and arrive and are accepted correctly.Therefore I will not pursue the DVI-D further.
I needed to specify ConnectedMonitors "CRT" to force the analog signal even although I had a vga cable connected. That was contrary to what the readme said. Without the option the log showed that a DFP was thought to still be connected.
My lesson here is that with a DVD-D connection never try to belittle the almighty EDID
It has powerful friends.