Bigbanana, nice diagrams!
I think the OP was talking about low level HU outputs rather than amplified ones though?
Agree, an amplified output is usually not solidly referred to ground on one pole, as the speaker itself has no need for a ground reference, only signal differential.
However, the low-level signals need a reference level in order for the signal to have some relative amplitude / phasing between amplifier stages.
Either, for low cost, you use a 2 wire connection with one grounded as ground continuity and signal wire in one. (i.e. RCA)
Or for better noise isolation you isolate the A & B wires from ground, and use a 3rd wire/screen as ground to give ground continuity between devices.
Agree on you isol diagram, just needs a grounding connection shown from the RCA outer portion