Rolf I'll riff on your questions without, at least for the moment, any specific citations other than to say that everything I'm about to refer to is in the PDs. For me, the PDs have been tricky. At the beginning they sounded rather obscure, but as I have read, paused and returned to them over various intervals they seem to be pretty straightforward, although incredibly insightful. They require time and percolation.
First, living with chronic pain is a case study in the way in which it is unhelpful to imagine that there's a neutral state. The neutral state is, to me, a product of laziness and lack of rigor. When I've found myself thinking that I'm in a neutral state, I've repeatedly noticed that it really means that I'm not paying attention. Once I pay more attention to my feelings, I invariably notice subtle pleasures and pains at various places in my body and mind.
From this it becomes evident that, as stated in the PDs, pleasures and pains have intensity, location and duration. I often find that some of my parts are in pleasure or pain, but others not so much, or quite the opposite. And a feeling in one part might be drowning out a feeling in another part. A feeling may spread from one part to others: sciatica that comes and goes may prompt mental anticipation of pain, which causes muscles to tense, which lights up the sciatica... &c. (This could be thought of as an example of "pain v suffering".) Or a subtle feeling of pleasure might spread, thus increasing in location and perhaps duration.
Eventually, it becomes apparent that none of the feelings are constant, at least up to an extreme point as per PD04.
For me, part of the beauty of Epicurus' analysis of pleasure and pain is that it is exceptionally nuanced and provides the Epicurist so much to work with. We can offset various pains by noticing and dialing in to pleasures in other parts, and we can work at increasing these pleasures even if we can't seem to diminish the pains. We can seek pleasure strategically when we understand what, for each of us, provides the greatest payoff in terms of maximizing our pleasure and minimizing our pain. Sometimes this takes time. Lots of time. But we humans are intricate instruments, not the golems that Cicero and the Stoics imagine us to be.