I couldn't care less what FFRF's letter says. Especially when they're wrong. I care about case law.  The Freedom from religion is not whom I would get any information on as to what discriminates against a religion, they wish to discriminate against ALL religion. They are a biased source.

You are accusing me of a limited knowledge of case law, but you misrepresenting Greece v Galloway. Greece v Galloway  supports Kenai. The city of Greece had a very similar policy. Greece v. Galloway held that the City of Greece had NO obligation to go out side its borders to find minority faiths.  It also did not say a word about Atheism.

"The District Court found no authority for the proposition that the First Amendment required Greece to invite clergy from congregations beyond its borders in order to achieve a minimum level of religious diversity."

As long as the Borough allows invocations of all congregations within its borders, their policy is legal.  
Greece V. Galloway support Kenai not you.