The NEP currently does not say who this is meant for. Would you expect libraries like SciPy to adopt it for example?
The NEP also (understandably) punts on the question of when something is a valid duck array. If you want this to be widely used, that will need an answer or at least some rough guidance though. For example, we would expect a duck array to have a mean() method, but probably not a ptp() method. A library author who wants to use np.duckarray() needs to know, because she can't test with all existing and future duck array implementations.
I think this is covered in NEP-22 already.
It's not really. We discussed this briefly in the community call today, Peter said he will try to add some text.
We should not add new functions to NumPy without indicating who is supposed to use this, and what need it fills / problem it solves. It seems pretty clear to me that it's mostly aimed at library authors rather than end users. And also that mature libraries like SciPy may not immediately adopt it, because it's too fuzzy - so it's new libraries first, mature libraries after the dust has settled a bit (I think).
As discussed there, I don't think NumPy is in a good position to pronounce decisive APIs at this time. I would welcome efforts to try, but I don't think that's essential for now.
There's no need to pronounce a decisive API that fully covers duck array. Note that RNumPy is an attempt in that direction (not a full one, but way better than nothing). In the NEP/docs, at least saying something along the lines of "if you implement this, we recommend the following strategy: check if a function is present in Dask, CuPy and Sparse. If so, it's reasonable to expect any duck array to work here. If not, we suggest you indicate in your docstring what kinds of duck arrays are accepted, or what properties they need to have". That's a spec by implementation, which is less than ideal but better than saying nothing.