To answer your questions..
1. The minimum requirements depends on lot of other factors as well like how often the data is going to be refreshed and number of scheduled reports that are going to run and so on. What you mentioned is the minimum requirements, you need to put in more rig if you are using it for production. You can read about it here Before you install...
2. Tableau does not recommend using one node for production, keep the process distributed and setup fail-over. You can check this out to know some light on it Choosing the right Tableau deployment strategy | The Slalom Blog
3. It should work far or less closer to all databases. But it highly depends on the type of data, no of rows and operations made. Extracts also affects the performance level depending on how they are used.
4. If those users would not require to edit the worksheets, then they wont be needing the license in your portal, But if you are using something like Active directory, they would require licenses at Tableau server for working.
Hope it helps, feel free to post more questions.
Thank you very much for your advice and the links you've put as above.
Happy to help. Please mark the answer as complete if you feel the information helped you. Or post your questions and we will be happy to help you out in whatever way we can.