They'll probably answer your emails by the end of the week (and I understand not having the passport is frustrating, but unfortunately there is not much you can do to speed hinges up as they indeed don't take phone calls).
If you go there directly you will have to convince someone at the reception desk to call them and have them come down and talk to you. This might be difficult but since you have good arguments it is possible.
Bring your letter to show the guy and explain that you got that note from the EU Treaty Rights section saying they are sending back your passport, but they forgot to include them and need the passports but you can't reach them by phone or email (and I know the situation is stressful but at least at first try to be calm and polite
). If the person there is reasonable and if you insist enough they will call them to check if you can have them come they should be helpful (I have heard of someone before who was able to get their passport back from them because they needed it to travel).
Finger crossed for you not to come across a grumpy and unreasonable clerk downstairs.