Yes, the digital switching on the i-bus (or k-bus) wire is coupling to the audio signals.
Yes, shielding of at least the audio signals is absolutely required.
The first level of shielding is acheived simply by twisting the audio + and audio gnd together, known as twisted pair. This can also be done for the i-bus, it can be twisted in a group of three with the +12 and GND.
Real shielding is better.
So I need to ask ... why didn't you just snip the original usa-spec supplied cable & wire that into the HU connectors like we have been discussing? I never imagined that you were not using the original cable. lol