The last suggestion might be an idea. There is no proper logic to assume opening the sample editor is the more right move to do when pressing numpad 0.
Don’t forget you can also press F4 to get the sample editor up front and F3 to get the instrument envelopes up front (if you have the default screen area presets stored).
The fact that you only have to memorize those keys specifically makes a three toggle for the numpad a bit clumsy.
I mapped my F1-F7 keys differently than the default.
The Numpad0 addition is not only to make access to this easy.
The thing is this - working with many instruments brought me to the habit of pressing Numpad0 whenever I want to edit the instrument - since most are VSTs.
So, when I do that, and the instrument is in fact a sample - in previous versions, nothing happened.
Now, I think it is better - there is no need to memorize anything since I do it automatically and without thinking, but sometimes the sample you are editing needs changes in the instrument pane, and sometimes in the sample pane - in other words, editing a sample based instrument has not one but two editing areas.
So, thats why this toggle may make sense - i mean, if you are not a part of the group of people that uses the numpad0 a LOT, then it wont bother you, but if you are one of those people - it will probably only make you more efficient.
Press Numpad0 when a sample is highlighted, and see how it will make a total sense for you to press it again to move to the sample editor.
I agree though that the logic is not straight forward, unless you say: Numpad0 = Edit Instrument.
That became the new logic because numpad 0 made the VSTI editor appear when the instrument contained a VSTI, but it did nothing when a sample based instrument was selected.
The most logical equivalent of the VSTI editor is the instrument editor when it regards sample-based instruments.
Hence that you now view the instrument editor when you press numpad 0 on a sample based instrument.