You should get both channels out of one speaker if the balance is pushed all one way, its not just a volume control.
Yups it is - it doesn't remove any of the stereo audio, it's literally a volume limit for left to right.
I stand corrected. I only say that as none of my HUs over the years have done this from factory installations, cheapo Argos items to some half decent items .
Anyway, I am glad that you got it sorted.