I'd also like to ask if you can accept mush/apples as "bank account money" and add mush/apples as "interest" weekly or monthly to that "bank account." (In exchange, the bank uses that "bank account money" to give loans or whatever.)

There aren't "Bank Accounts" on WP. The closest thing is a Mushroom Hole [ Den > Secretary Bird > Mushroom Hole ] which doesn't give interest.