> We never got the letter for the renewal

Wait, we get a letter? Never got a letter for any renewal, so I am confused.
That said: you should be able to get an extension for your current permit, 3 months is a realistic timeframe for a new permit if you can also renew it on monday.

Let's hope the rückreisevisum is not too expensive or complicated..

Basically, they wont eat you alive, people make mistakes and whoever you encounter on monday also made mistakes. It's not bad to just admit your fault (dont mention the letter you were expecting, this could be awkward if they dont send these out... and it could sound like you want to shift the blame). Just tell them you fucked up and you are sorry, but you are here to fix this issue, and then let them explain what options you have, and pick the most suitable.

Side note: It's always, always good to know what options you have, but if you show up like a wisecrack and YOU lead THEM through the process, you'll have a bad time (probably anywhere, not just with them). If you should have someone that has a bad day and is not cooperating, kindly ask for the rückreisevisum, an appointment to renew your B permit or get the C, and an extension for your current B permit until the next appointment and the new permit is ready.

This is a special "monday" hint..