>> I find it almost inconceivable that an accusation of anti-semitism is not defamatory, particularly of a person holding some public office. . . . It is beginning to look as if the USA searches first for the true meaning of the words used, and then addresses that found meaning, if I correctly read the above.<<

I've been pondering. The answer is (of course) "yes and no." US defamation laws derive from British common law, and for private plaintiffs complaining about "private" issues (i.e. issues lacking public a interest component), I think you would find rather minor distinctions from the law as you know it (including a presumption that defamatory speech is false – giving rise to your concerns that your clients be able to prove up their defenses). However, if the speech has a public interest component, the First Amendment changes everything drastically. If the speech has a public interest component, and the plaintiff is a public figure (as Greenbaum), the bar is set even higher.

There are two major cases that explain it (and googling should take care of it). The first is New York Times Co. v. Sullivan, 376 U.S. 254, 270 (1964), which clearly set forth the Constitutional override of common law by the First Amendment and the "profound national commitment to the principle that debate on public issues should be uninhibited, robust, and wide-open."

The second is a case called Philadelphia Newspapers, Inc. v. Hepps, 475 U.S. 767, 769 (1986). The Supreme Court expressly reversed a common-law presumption that defamatory speech is false, and changed the allocation of burdens that you are discussing. (If the plaintiff is a public figure and raises an issue of public concern (as with Greenbaum), she must prove scienter (knowledge) and falsity. If the plaintiff is a private figure but raises an issue of public concern, then the plaintiff must prove at least negligence and falsity to recover actual damages. If the plaintiff is a private figure and raises no issue of public concern, then the Constitution does "not necessarily force any change in at least some of the . . . common-law landscape." – Hepps.)

But even beyond allocation of burdens, I honestly think the trial judge will rule that the speech in question was not defamatory in the first place, as I wrote above.