How can missed prayers and fasting be made up for, and what is the best way to ensure one is not held accountable for them upon death, and are all bank loans unlawful, even if they are for building a house?

According to the majority of scholars, one must make up for prayers missed intentionally, and this is the more cautious approach. You should strive to achieve certainty or a strong presumption of having fulfilled your obligation. The same applies to making up for days you missed fasting in Ramadan (definition).