Is the action you took -of preventing your father from mortgaging the family home to a bank that deals with interest- Islamically permissible?

This action is permissible, and you will be rewarded for it, Allah willing, because you nullified a forbidden contract and saved your father from explicit usury, which is agreed upon by all scholars. This is especially true since it was done in a way that did not change your relationship with your father or hurt his feelings. Advising parents gently and alerting them to their mistakes is not disobedience; rather, it is a form of changing evil and saving one's family from the Fire, as Allah Almighty says: (O you who have believed, protect yourselves and your families from a Fire) [At-Tahrim:6]. Qatadah said in its interpretation: "You command them to obey Allah and forbid them from disobeying Him, and you establish Allah's command among them, ordering them with it and helping them with it."