Playing a single note confers no meaning. It's only when subsequent notes are played that a context emerges, and music gains its emotional qualities.
I would say it makes sense - perfect pitch or not - every key has a different relationship to your own voice, which makes it feel different.
In most jazz music you will have the lead sheet so you don't really need to know anyway. Besides that, you don't really need to calculate anything if you have relative pitch. If you practice enough it's instant. If I hear a note or chord followed by another note or chord I can tell you their relationship in real time. Anyone who has done enough ear training can. In fact, it's one of those things where you either know instantly or you haven't practiced enough. I was never really in an in between state.
I've never experienced any correlation between pitch sensitivity and improvisation skill in groups of trained musicians.
I play a lot of improv and normally it takes no more than 2 notes playing along to determine the key of the piece and relative pitches are very learnable.
Losing some high pitch hearing is bad enough.
Not necessarily the intervals between any two notes, but the note of interest in relation to the tonic. But still may require the relative position to be mapped (offset) to a specific note.
Not at all. You just hear interval.
AP helps improvisation (mostly keyboard and strings, less so brass, and even less so woodwinds, I can explain later). Also, ease/speed of composition, allowing you to focus on the creative aspect of composing, or just simply composing more.
You can become a great improviser with relative pitch, but it is much harder, as you have to calculate the intervals between notes in real time, whereas AP spits out the exact note automatically for you.