1. Between "not very well" and "not at all". Have a look: https://en.wikipedia.org/wiki/Middle_High_German 2. No idea. 3. As with all variations or dialects of what is basically the same language, it depends on a lot of variables, chiefly a) the level of education of both hearer and speaker and b) the level of willingness of both hearer and speaker to understand and be understood. Swiss Germans usually can and kindly do switch to some kind of modified High German when speaking to Germans from Germany. However, if a Swiss German film is shown on German TV, it gets subtitles.

1. Middle High German, such as in the German parts of the Carmina Burana, is surprisingly close to modern Swiss German dialects, as these dialects have skipped most of the sound changes. that have occurred since. So it should be easy for a Swiss Person or somebody from southwestern Germany, except maybe for words that have changed their meaning over time. For any other German speakers it will be very hard. I remember reading some Middle High German texts in school. We needed a lot of explanations to understand the texts, but after some practice it was possible. 2. The Prussian dialects are almost extinct after the displacement of the people who spoke it after WWII. I just looked up "Hochpreußisch" and "Niederpreußisch" on Wikipedia and from what I read there, these dialects should be as understandable as most middle and lower German dialects. But I know nothing about the "old" variants of these dialects. 3. When using Standard German (Hochdeutsch), Germans, Swiss and Austrians can understand each other without any problem. They will recognize a Swiss, Austrian or northern accent and maybe stumble upon a few words that are used differently, but that's about all. When using their dialects, it is very hard for them to understand each other. This only works after a longer time of practice. In my case, it took me about 2 weeks of immersion to understand a first Swiss dialect. And later when facing dialects from other parts of the country, I needed a couple of days before I could follow a normal conversation. The same should apply for any other dialect from Germany or Austria. I guess, by Hungarian you don't mean the Hungarian language, which is something completely different, but the German dialects of Hungary. I haven't heard any German from that area, but I know some people who emigrated from Romania ("Siebenbürger Sachsen" and "Banater Schwaben"). Their dialects are yet somewhat different from those mentioned above, but you don't hear them too often.