1. Guest User on Tableau Online -> Feature request.
How much users will open the view ?
What you are looking for is "Guest Access", and this is only available for Tableau Server with Core-based licenses. -> Guest User
For Tableau Online, there is no guest access, all users accessing views and dashboards published to Tableau Online must be authenticated.
As a workaround there is also "Tableau Public", it's a free service for publishing views that you can share publicly. (However all data should be in a flat file, like excel, json, extract etc.)
I recommend this article to have a good understanding between Tableau Server and Tableau Online.
You can request a call with one of our Sales/Account Rep, they can also help you to find the right choice
2. Tableau Online -> Tableau Server
Yes, you can switch from Tableau Online to Tableau Server, but you will need to open a support ticket.