Is it permissible for me to assist my accountant brother with tasks that might involve usurious accounts, or with the maintenance of his office, given his apprehension about falling into what is forbidden by Islamic law?

The consideration in teaching knowledge is for whom will use it. If you know or predominantly believe that your brother will use it for something forbidden—such as performing calculations for usurious interest—then it is not permissible for you to teach him, because that entails aiding in sin. If you do not predominantly believe that, then the default is that teaching it is permissible. As for repairing the phone, if it belongs to a usurious institution, then do not repair it.