Even if it turns out that there is no rigorously definable one-dimensional measure of valence we still need to search for physical correlates to pleasure and pain and find approximate measures to use when resolving moral dilemmas.

Regarding the response to (6), why don't you want to maximise hedons? Having a rigorous definition of what you are trying to maximise needn't mean that what you are trying to maximise is arbitrary to you, and that pleasure is complex (or maybe it is simple but we don't understand it yet) does not imply that we don't want it.