Department member forwarded complaint about professor to that professor
While I can understand why the advisor reacted this way, why would a department member forward an email explaining a problem between a professor and student to the professor?


Is it the senior professor's formal task to handle complaints about or escalations from the advisor?

If yes, the senior professor indeed acted fairly unprofessionally, although it is a reality that in most cases the person that you complained about will learn about who complained about them as a matter of practicality (see some other answers for why this is).

If no, then, well, I am not sure what you thought would happen. Then you sent a random (presumably busy) senior person a mail about an issue that is in no way relevant to their tasks, and they did the only thing that makes sense from their perspective: glance the mail and forward to the person who is actually in charge for the request.

Look, I get that the first instinct when you get an unsatisfying decision is that you "would like to talk to the manager", but this is not how academia (or, really, any business) works. Department administrators delegate specific types of decisions to individual faculty members so that they don't have to deal with them one-by-one anymore. This is arguably not helped by the fact that faculty in academia tend to be very wary to step onto each other's toes. Hence, you will need to work with the undergrad advisor on these issues, even if you don't care for him much.