Is it considered apostasy for someone to say "Insha'Allah bi Rabbi" (God willing, by my Lord) mockingly, intending not to do something, and is it permissible to combine the ritual bath for janabah with the ritual bath for apostasy?

Saying "In sha' Allah" (God willing) with the intention of not doing the action does not necessitate apostasy, but it is not appropriate. Washing an apostate if he embraces Islam is not obligatory, unless he committed something that necessitated ghusl (ritual bath) while he was in a state of disbelief; in that case, the ghusl would be for the state of janabah (ritual impurity). For those who believe in the obligation of washing an apostate and every disbeliever if he embraces Islam, one ghusl suffices for both janabah and entering Islam.