Just for the sake of thought, not to disagree with your points:
- the state of no pain is the absolute pleasure (or is it "the highest" pleasure? and should we add for completeness that the state of no pleasure is the highest pain?)
- pain and pleasure cannot be mixed (I see no issues there, unless it helps to say that they can "coexist in separate parts of the experience, but not mix")
- even at rest and without intense stimuli we can feel pleasure (given that pleasure is not defined as requiring stimuli at all, but as "agreeable feeling")
- weak pain can be endured, intense pain is not permanent (there is limit to absolute pain). (to "the highest" pain?)