I'm still getting a lot of differences between reported widths, so I'm going to work out seat width from cabin width directly.
Assumptions for all aircraft:
Aisles = 16in, regulatory minima
Armrests = 2in and are counted even on the sides (e.g. aisles or window also count)
Distance between window seat and cabin side wall = 1.5in
So for 77W in 10-abreast, cabin width = 231in, take away two side walls, two aisles and 13 armrests = 170, divide by 10 to get 17.0in
Remaining calculations
I don't think anyone actually has a 19.5" A350, 19.11" 77W@9 or 18.4" A330 seat width, because the aisles will be wider for everyones' convenience.
But that's the whole point of my exercise - start with cabin width and cabin width alone, and assume everything else is the same across different aircraft. Finally an apples vs apples comparison.