NRS towers are easy to swap. If you have 8", buy a set of 10", if you only use them for this trip, they're easy to resell. Even if you lose $15 on the deal, it's worth it for a trip.
Ever row with a stern frame?
Center pivot point isn't as critical as one might think. The boat pivots around its center of lateral resistance in the water, generally near the center of mass. It just "feels funny" when you're not pushing on that exact spot...but the boat still performs. I don't think 6" makes near as much of a difference.
I hear you on the bad back, that's a consideration worth working around.
Does she have a seat on her own boat she can bring over to yours? Maybe even rig it so she's sitting on it when she's your bow passenger?