The keeping properties of a developing agent can be greatly incresed by the addition of another. It might sound surprising but Ilford sold D&P tank developers with Metol, Hydroquinone and Pyrogallol, designed for high volumes and replenished. The Pyrogallol acts as an oxygen scavenger.

PPD also forms a complex with some developing agents, the most common being Meritol which is a PPD/Pyrocatechin complex. It may well be less toxic as a complex although the commercial developers I used which cointained Meritol did have a warning label even in the late 1960s.

Womens (and the odd vain mans) hair dyes use(d) PPD derivaties, most have changed to safer compounds but not everywhere.

Some of these developers gave exceptionally good results and in a way they were the magic bullets when they were first introduced. Modern films are less affected in terms of fine grain by choice of developer, old style films though could give grain like golf balls with the wrong type of developer and excellent fine grain in one of these off beat fine grain developers, far better than most fine grain developer from Kodak, Ilford Agfa etc.

As to why p-aminophenol in a low pH developer well Ilford used it alongside Metol in their early super fine grain developer sold in the 1930's.