I don't know what they mean in GTD, but here's what the words mean to me. Defer means leaving it for later (or leaving it for either later or never). Incubate means needing to think about it, perhaps at a subconscious level, before taking the next step. If I write an email on a sensitive topic and wait until the next day to re-read, edit and send it, that's incubating. It's not because I don't have time right then to do it; it's because I want to get some sleep and look at it again in a different frame of mind, or even think about it in my dreams to get a different perspective (if that' s how dreams work). On the other hand, if I have a bunch of emails I need to write and also have some other important work to do, and I leave the emails for the next day so I have time for the other work today, (there isn't time for everything at once), that's deferring. Someday/maybe can be considered incubating because you're leaving time for your feelings etc. to decide whether the things are really worth doing or not.
Inability is an abstract thing involving comparison with alternate universes; it cannot be experienced.