Screen brightness isn't something you can manage through an MDM and single app mode restricts the use of control center so you can't manually dim the display while in single app mode. You could set the brightness first, then put the device into single app mode.
What about putting the device in sleep mode? It seems like I cannot but the device in sleep mode when single app mode is activated? Is there a work around available?
On a test iPad that I put into single app mode I am able to press the sleep/wake button and put it to sleep and then wake it up again. What iOS version are you running on it? Im running iOS 12, but I wonder if iOS 11 might handle that differently.