A simple uestion, I hope!
Can I power the Mega with the power plug AND connect the USB plug for programming and debugging. The power is provided by an approved 9V adapter.
I could remove external power for the above purposes, but not easily. The Mega is hard wired into the project.
Yes, unless there is some strange ground loop between the 9V adapter and the power supply of your PC used for programming. The safest way is to use a laptop for programming on battery power. Never plug in its power supply (or any other plugs) while plugged into the Mega.
Usually it isn't a problem.