If you are certain that the ground connection between the two is good then I think there is a possibility that your DueX5 has an intermittent fault, and you should ask your supplier to replace it under warranty. https://www.duet3d.com/warranty
Yes, forum user W3DRK does. Use the Chat option to send him a message.
Nice!! Gotta love that flat map.
1.8 has 200 steps per revolution and 0.9 has 400 per rev.
Double your M92 for the axis you are installing these.
M92 X80 Y80 Z80 would go to M92 X160 Y160 Z160 for a delta.