frodebro

Both the effects loop and the insert in the Marshall are located essentially in the same place in the circuit-meaning between the preamp and power amp.  I'm guessing that the loop is switchable and the insert point is not.  If this is the case, then just use either one of them as a standard effects loop for your 4CM connections, but don't muck about trying to utilize both loops as it won't gain you anything except a mess of unnecessary wiring.

The way I have mine set up right this minute is loop 1 send to the alternate input on the rear of the KPA, and the main outs to loops 1 and 2 returns.  I use a mono send block and a stereo return block.  I have also run it with S/PDIF cables, the connection there is Helix Path 1 output set to S/PDIF (which goes to the Kemper S/PDIF in) and Path 2 input also set to S/PDIF (Kemper S/PDIF out).  This puts everything you place on path 1 in front of the KPA, and everything on path 2 after it. 

This is the most important question.  Plugging a mono cable into the headphone jack WILL short one channel to ground, which could damage the headphone amplifier.