Hi Svend,
reports say increased pH as result of hydroquinone oxidation...so, full bottle shouldn't change pH, I'd guess (only if was more interested in chemistry at school
, can HQ react with oxygen form water?).
Some people swear that metol only developers give better edge sharpness than metol+HQ - as for my experiences, D-23 1+1 is very sharp, but I doubt that there is some difference compared to D-76 (at least up to 30x40 print from 35mm).
Also, without superadditivity, metol only developer should build high density areas slower, providing less contrast (I have noticed and use this with D-23).