Does the legal ruling differ between the serious and the joking person regarding describing an original disbeliever as an "innovator"? And does the rule "Whoever does not declare a disbeliever to be a disbeliever is himself a disbeliever" apply to him?

Most likely, the statements mentioned fall under the category of prohibited jesting. It does not appear that the speaker intends to lift the ruling of disbelief (kufr) from an original disbeliever. Therefore, the rule "whoever does not declare a disbeliever to be a disbeliever is himself a disbeliever" does not apply to him, because if he were asked about the religious ruling concerning an original disbeliever, he would answer that it is disbelief.