Comparing US to UK on a matter of this kind of subtle differences can be ticklish.
To begin with, since most of the US Army Helo Pilots are Warrant Officers, your cited UK "officer/other ranks" structure won't match.
While I am pretty sure that the Army requires the pilots to be qualified in both seats, it has been so many years since I was engaged in that kind of conversation with an Army colleague (about 20 years ago as we were checking out a few things Fort Rucker was doing that we might want to adapt in Milton, FL) that I will not attempt to speak for how the US Army does that.
To throw another curve ball at you (or a cricket spin bowl) when I was an aircraft commander in the SH-60B, what seat I sat in made no difference. I could sit in either the right seat (pilot station) or the left seat (Tactical station running the ASW/ASUW/EW kit) but I was still the Helicopter Aircraft Commander (HAC) regardless of which seat I occupied. However as that was USN and not US Army, that hardly helps your Apache(UK) to Apache(US) comparison.
I don't think "which seat you occupy" governs who the aircraft commander is. (Again, been years since I flew in an Army aircraft with an Army pilot alongside, so I just feel out of date ...)