I thought the noise was all the time on the large chainring? Is this correct?
Yes, it is but I'm going to check it again on Thursday, just to be sure.
AFAIK the correct Shimano and KMC chain widths will be the same for your 11spd system. In which case I don't see how swapping the make of chain will have a different result.
Find some way of safely getting at least the rear wheel off the ground (even if someone holds it for you) and rotate the cranks slowly and, using a torch if necessary, you should easily be able to spot where the chain is hitting the FD cage.
I intend to set up some sort of makeshift rig to hold the bike off the ground so I can do this
Do that and you'll be half way home.