[ApK]

Listen Audiobook Player has configurable actions for clicks, and multiple-clicks, of the headphone buttons that accomplishes everything you describe.

It's not a feature I've used much, personally, but it wouldn't be there if someone didn't get use out of it, I suppose.

Are you thinking of making a player app with these features, or are you thinking of making a utility services that intercepts the buttons and enables these feature in any app?

If you're thinking of making a player app, you'll need to worry about a lot more than this one feature to make it at all competitive, but good luck. It's always nice to have options.

If you're thinking of making an enabling utility, I can see how that might be interesting. (Though I'm not sure how it could be possible....)

ApK