It is lovely to discover the cultural differences as well as the German language.
Hi Jack, being German myself I feel qualified to comment on some of the aspects you mention there.
Nowadays you are allowed to cut the potatoes with a knife. The rule comes from a time, when knives changed color from the starch. It's outdated now.
Most people use their hands to eat a sandwich.
Some of the foods you mention are only characteristic of a certain region. For example, I know many Germans who either do not like Sauerbraten or never tried is, even though it uses beef nowadays.
as a german i have never heard of some of the things you wrote there. like not cutting potatos with a knife or generally eating sandwiches with fork and knife. i know people who do eat sandwiches like that, but there is no rule for it.and how am i supposed to cut the potatos if i am not allowed to use a knife? Sauerbraten at our house is always beef. never heard of it beeing made from horse meat. Plus the foods you mentioned are all regional dishes. you can only get Käsespätzle in southwest germany for example. but there are other customs wich would be more worth mentioning, like the tradition to bring a small thank you gift for your host if you are invited to someones home.
If you know enough of it, or are currently still learning about it. Adding other cultures that speak German would be cool too like Switzerland or Austria. I'm currently learning German due to the Swiss as I plan to live there in the future, but I do have a fascination for the culture of Germany.