Yes, and no. The same is true for Switzerland (Swiss German) and bavarian german. There are some other horrible accents in the German language but these are the most difficult to understand, even if you're a native speaker. Native speaking Germans may have an easier time understanding Austrian than a non-native speaker, but I wouldn't say they could replicate it, or even understand every word. It's kind of like an American and a Scot speaking English. The basic language is the same, the accent and certain local terminologies are different.

They are more than different. I have lived in Austria for 14 years. But I still do not understand what my co-workers are talking. Of course not zero but less than 50%. I have never heard my co-workers were talking in standard German except that they were talking with German people who speaks standard German. But it sounds unnatural. Wherever I would go, almost everybody would say that I speak perfect German, especially foreign people, that they can understand my German well. During work, they would speak more technical terms, I can follow their talk. But maybe it depends on the situation you would be in. Even in Austria, people would talk in standard German from time to time. It depends on which city you would be in or company you would be working for.

In my Opinion, it depends on where you are from. I come from bavaria so for me it is easy to understand Austrians since it sounds very alike. I have way more difficulties understanding someone from the north of Germany. And speaking about Swiss german, that is almost not understandable at all.

Keep in mind that, unlike the 'dialects' in English, Austrian German and German with an Austrian accent are two different things. Austrian German is very hard to understand, but all Austrians speak High German and speak it with only a small accent. I think the situation is more similar to the situation with Arabic dialects and MSA than the situation we have in English.

Yes! And I have to say that this topic is huge. To get yourself some insights I recommend that you take a look at the Wikipedia item under https://en.wikipedia.org/wiki/Austrian_German Have fun!