If the theory is that the Phelpses were disturbing the peace/disorderly conduct, they could have been charged with that, the argument being that somebody shouting something noncontroversial -- "I think apple pie is wonderful, if eaten in moderation!" -- in the same circumstances would also be a discon, and obviating the Constitutional question.

Not necessarily true. We've got 'fire' in a crowded theater issues, there. Shouting 'water' or 'air' or 'earth' in a crowded theater isn't going to get you charged with the same thing as 'fire'. The political nature can't be considered, but the inflammatory nature can be, I'd suggest. (And yes, I know how bad that particular metaphor is, but it's the one we've got.)