I also know that if a Catholic marries a protestant in a protestant ceremony that a lack of form is processed.
Not necessarily. A Catholic can receive a dispensation from form to marry in the non-Catholic’s place of worship. So, whether or not there is a lack of form issue in this situation depends upon whether or not the Catholic received the proper permissions and dispensations pertaining to a mixed marriage. A Catholic could very well validly marry in this situation.
What happens if the Catholic converts to the protestant religion and is married in that tradition, but divorces, returns to Catholicism and wishes to remarry (somebody else)? Is an annulment or a lack of form required?
A Catholic is always obligated under canon law to obey the laws on marriage. To complicate matters, for a time between 1983 and 2010 there was a canon law process called “formal defection” in which the Catholic could, in writing, defect from the faith and be received by the bishop. In such a case, the person could then validly contract marriage outside Catholic form.
So, the answer to your question is “it depends.” Very few people ever did the “formal defection” process when it was available. It is no longer available, Pope Benedict removed this option from canon law because it created many problems for tribunals trying to determine if a person had indeed “formally defected.” Merely leaving and going to another religion is not “formal defection”-- it was an actual process that had to be followed.
So, if the person just stopped practicing Catholicism and took up this other religion, married without proper permissions and dispensations, then yes it would be lack of form. If this person was one of the rare people who followed through with a formal defection, then their marriage might very well be valid.
The protestant religion is Mormon if that makes a difference.
The Mormon religion is not a protestant religion. They are non-Christian. They do not have valid baptism and reject the Trinity. The Catholic Church has ruled their religion to be non-Christian.