Uhh, Hustler v. Falwell lends support to your conclusion only if the soldier in question is such a public figure...
I think that he would qualify as a public figure, insofar as he was an employee of the federal government. In any event, the protests were not directed against him specifically; they were directed against the public policies of the federal government of the United States. He was simply cited as an example.
I don't see why speech needs to be parody to qualify for this higher level of protection.
So you're basically arguing that any employee of the federal government, no matter how insignificant, is public enough not to warrant the same protection from distasteful speech as the private citizen?
I'm not really saying that for speech to survive, it need to be parody of a public official, I'm saying high value speech generally gets protected, on the other hand, low or no value speech--advertisements, hate speech with a malicious or reckless mental state, remarks intended to incite a crowd--doesn't get, nor does it deserve extensive protection.
And I find it hard to argue that Westboro's speech had any sort of significant political value...I think its worthless...and thus, time place and manner (and any sort of Tort related claims) regulations are more than appropriate.
This isn't a heckler's veto as others have alluded to, this is hateful speech that deserves no protection in legitimate discourse.