is dumping a Phenidone-Ascorbic Acid developer less hazardous for the environment?
Depends on what the main problem is you're looking at. In general, the answer "should" be yes - I say "should" be, because these PC developers are sold as 'eco', so let's hope the net effect is indeed more 'eco' than for other developers.
You may notice a little cynicism in my words, which is due to the fact that determining the 'eco-ness' of a product is insanely complex, and it's virtually impossible to say if one developer is necessarily much better than the other based on its constituents alone. You
can say something about it, but validating such claims is virtually impossible.
https://en.wikipedia.org/wiki/Life-cycle_assessment
To come back to your question: you limit the scope to 'dumping', but this still leaves the question how and where it will be 'dumped'. Do you mean chucking the used developer into the backyard, into nearby woods or fields, into a sewer system, into a septic tank or at a local waste disposal station? You'd have to consider these options and then work out for every constituent of the used developer how these materials fare after the dumping is done - do they linger about and dissipate through the environment? Do they seep into deeper soil layers and perhaps into the water table? Are they degraded by microbial life, and if so, how fast and by which species, and how likely is it these species are around in the (eco)system where the dumping took place? What is the energy balance of the disposal method (think of e.g. incineration of liquid waste and the logistics involved) and any byproducts and side-effects?
And that's just dumping. Now look at the actual makeup of the developer, how the ingredients are produced and what the environmental (and perhaps social??) impact of the production and logistics of those ingredients are.
At various places in the process you'll have to make judgement calls about how to weigh effects. If you compare a borate-based PC developer to a borate-free MC developer, how do you weigh the potential impact of borates vs. the difference between phenidone and metol? How do you weight the energy impact of both options vs. their environmental impact? Which dimensions of environmental impact will be deemed more or less important? The only truthful answer here will be that a quantitative assessment will always fall short of being accurate, so you're stuck with a qualitative or mixed method assessment that turns out to be quite complex to answer.
The question you ask seems to simple, but the truth is that it's impossible to answer.