These questions seem like ones you should be able to answer yourself as we don't know your environment.
1) Do you have Tableau Server? Or are you publishing dashboards on Tableau Public? You should know this. You can also embed dashboards, these links may be helpful:
2) Dummy ID: If you have Tableau Server, you can enable "guest" account. Rest of question 2 you or your customer should know the answer.
3) You or your customer should know the answer. Typically, on a Windows server anti-virus and firewall are enabled to provide some protection.