I would ask, "In Epicurean theory, is ANYTHING in life (even desire) always pleasurable except pleasure, or is anything always painful except pain?"
There is no pain or pleasure apart from the two feelings. I'm not saying you are intentionally, but your phrasing makes it sound like there is some abstract absolute thing called "pain" and "pleasure." There is no pleasure apart from the feeling of pleasure; there is no pain apart from the feeling of pain. When a person is experiencing something painful, there is pain. When there is no painful feeling, there is pleasure. When a person is experiencing a pleasurable feeling, there is pleasure. Saying something like "except pain" paints the wrong picture. It's similar to saying "When I'm dead..." There is NO "I" when death is present. Likewise, there is no "pain" without someone experiencing a painful feeling.
I'd also add that there is no "desire" without someone desiring something.
As to "Is All Desire Painful?"... What happens if we expand it: "Is every desire accompanied by painful feelings?" since as I said, there is no standalone "pain" without a person feeling painful feelings.
If we look at some expanded synonyms or definitions that encompass the connotations of desire (epithymia; verb epithymeo) "set one's heart upon a thing, long for, covet, desire" there is definitely a perceived lack of something that someone wants to acquire. However, there is also the idea of aspiring to something, wanting to achieve something. In that case, is it a pleasant feeling to feel you are working toward a positive goal??
I don't know if my ramblings help the discussion, but this is an interesting question. I can see how the split happened in the discussion!