The three of us stayed for two nights at Sofia's place, and it was wonderful. Sofia was very flexible with regards to check in and check out (thanks!).
The apartment itself is very atmospheric and at the same time cozy. Lots of space, great sleeping places, no problems with wifi. Highly recommended.
It is 20 minutes walk to Hauptbahnhof, but Westhafen and Birkenstrasse metro stations are even closer.