The 76 seat arrangement wasn’t higgledy-piggledy as you put it: each half of the coach was a mirror image of the opposite half. One side in each half was not dissimilar to the original layout with 4 bays of 4 and an airline pair. The other side was one bay of four and 8 rows of airline seats. This was the only way to squeeze in an extra 4 seats per vehicle whilst still maintaining leg room. In terms of a compromise layout - and it did offer more airline seats, which some prefer (I do if on my own) - it was about as good as you get.
As for the current GWR layout, it is a disgrace. Mutilated power cars and mutilated coaches means it is a very poor shadow of what the HST was at its best.