You need to talk to a priest, pronto.
You are not married yet and you are already asking “what if we divorce” questions! That implies you do not see marriage as indissoluable. That is a problem all it’s own.
In the case of a Catholic marrying a non-Catholic, validity is based in part on whether or not the Catholic followed the Canon Law guidelines of marriage. If you marry in the Catholic Church or outside the churh with a dispensation, then YES the Church would presume your marriage is valid. And, NO, you would not be free to divorce and remarry. There would need to be a Tribunal investigation regarding validity.
If your fiance is not baptized, your marriage would have the character of being a “good and natural” marriage. It would not be a Sacrament, although it would be valid if you follow the Church guidelines for marrying. If your fiance were to become baptized, your marriage would then become a Sacrament.
You need to go talk to your priest– not your mother. Your mother is not a good source of reliable information regarding the requirements of the Sacrament of Marriage.