Any help would be greatly appreciated

Hi dolly 7, The shelving unit is constructed off the wall in front of the the cavity and sits on the floor. You would only need to fix it back to the wall so it doesn't come forward. It would not be good to suspend floating shelves on this wall unless it had a back panel going to the ground. See pic examples. Sliding doors were installed onto an archway first. Wall unit built in front of sliding doors as cabinet. For holding weighty items on adjustable shelves, try to keep the width of the shelves to a minimum i.e. 50cm wide x 18mm thick, otherwise they will bow. If you increase the width, you will need to put a shelf pin in the middle at the back. For floating shelves, back panel should go to the floor if wall has minimal structure.

Are these nib walls coming up from the floor or coming out from the wall? Are they merely side panels of the cabinetry, or structural?

No personal experience with these systems but I know they do make special acoustic rated gyprock style wall boards for this kind of situation. And acoustic paint coatings too of some sort. Would they help?