Your story is about a R2R and not R2R2R as you say you have done. I am hoping to do a R2R exactly as you describe it in 2-3 weeks, but I think 8-9 hours is enough. R2R2R as a dayhike is for serious athletes.
the general sense is R2R2R meant to be Rim-to-River-to-Rim... R2R is Rim-to-River and an overnight stay... I may be wrong altogether as I see 'Steve C's reply saying otherwise. I am not a local and my definitions are purely based on internet. I should have specified it explicitly in the my original reply.. anyway, yes you can do it much faster, just that we took lot of time to absorb it than to exert ourselves all out. ymmv. all the very best,enjoy the grandeur of the canyon, u sure sound like well prepared. if not, pl do.
