interesting....I cannot pretend to comprehend all of this, but it is very interesting.
I'm unclear about one thing though, if the increased contrast is expected (at least partially) due to the solution's greatly increased buffering capacity, would we not also expect increased contrast in the solution with both borax and boric acid? That is to say, wouldn't the solution with the excess borax and boric acid also have much greater buffering capacity than the nominal...and therefore also exhibit less (localized?) compensating effect?
Clearly, it is complex.
That might have happened were it not for the lower initial and therefore lower average pH.