Did you read the decision? Greece v Galloway clearly stated that invocations were acceptable only if anyone could give them. They were not subject to geographical issues, nor were they subject to review before hand. No religion could be excluded, no matter how controversial it was. 
The passage you cite is half of the story. The City could not be required to invite outside clergy to perform invocations, however as cited above in my post, the City had to allow clergy or laypeople of any religion to give the invocation. Something that the city agreed to wholeheartedly.

Galloway has already been used for two years in this exact capacity, and has been upheld each and every time as meaning this. Kenai would be excused for not inviting outside clergy in, but they cannot exclude laypeople or clergy, outside or not, from offering invocations on their own volition.