Is it necessary to make up for missed days of Ramadan with expiation, and does uttering words of disbelief in a previous period necessitate performing ghusl to re-enter Islam?

All praise is due to Allah, who has granted you repentance. You are not considered an unbeliever (kafirah) for having missed prayers in the past, nor for your mistaken belief that you would become an unbeliever. You are required to make up the prayers you missed, according to the majority of scholars. You must also make up the current days without any doubt. As for expiation (kaffarah): if you were unaware of the impermissibility of delaying the make-up prayers, then no expiation is binding upon you. But if you were aware, then in addition to making up the prayers, you must feed one poor person for each day you delayed its make-up.