Is a son informing a parent that a sum has been transferred to one of their accounts in a usurious bank considered backbiting (gheebah)? And is it obligatory for the son to object, or would expressing gratitude be considered an act of disobedience (ʿuqooq)?

The aforementioned statement does not constitute backbiting the parents. The child's thanking the sender of money to him, which was sent through one of his parents' accounts, has no relation to disobedience. It is incumbent upon the child to gently and politely forbid their parents from usurious transactions, without violence or abuse.