I'm no expert on NSI in general, or those dimmers in particular, but here are my thoughts.
Your three dimmers that are full on.
For these I'd strongly suspect the triacs, or back to back SCRs, are shorted.
This is often caused by folks plugging in a fixture, or extension cable, with a dead short.
They try it in one dimmer, it doesn't work, so they try it in the next dimmer.
The smarter users quit after blowing up two consecutive dimmers but some will blindly try three.
As to your second problem.
You mentioned disconnecting the board.
Did you also try disconnecting the control cable at the input to the dimmers along with any cable that may be looping on to other dimmers? You mentioned trying this with the first pack but did you try it with pack two?
If the control protocol was DMX, I'd be enquiring about end of the line termination. I've no real knowledge of Microplex so I don't know if it also requires termination.
Them's my thoughts.