If a DataSet has data that does not change the first message is a KeyFrame but when should the next one be sent?

In theory, the KeyFrame is never sent again but this behaviour defeats the purpose of the KeyFrame which is to allow new Subscribers to get a current snapshot of Data.

The KeyFrame count should be based on the PublishingInterval.

A KeyFrame should be sent every KeyFrame * PublishingInterval ms.
It should not depend on the number of delta/keepalive messages sent since the last KeyFrame since that is variable depending on whether data is actually changing.