#6 - I would take reliable advice on the day. If Tahrir is free of protests, there is no reason why you wouldn't go. If you're on an organized trip, they will make the judgement and if anything will probably err on the side of caution (which is what you would want them to do). Otherwise, you could check with your hotel, or even your embassy what they say the situation is like.
Re the Aswan trains, the Cairo-Aswan line is the main means of passenger transport for all long distance trips in Upper Egypt, and I wouldn't think that tourists have ever been a huge share of its passenger numbers - though I'm just speculating.
#7 - " It might be helpful to know that I will be visiting friends who have lived in the city their entire lives and should therefore know which places to avoid and so on."
Take their advice! They probably know the city and the situation much better than most if not all of those posting here, certainly including myself. Between whatever they tell you and I assume that travel advice is also available from the german foreign ministry, you should be able to work out how things are.
Lots of horrible things have happened over the past year and a bit, but by and large, life has gone on as normal in cairo outside the areas immediately affected by demonstrations - which are usually more or less the same ones (Tahrir, Muhammad Mahmoud Street, Maspero, and recently around the court buildings in Isaf and Maadi).
