1. No. Tableau Reader cannot consume "live" data sources like cubes and databases
2. No. You cannot create extracts from cubes using any version of Tableau.
3. Reader is free. However it won't do what you want it to (see #1)
4. Keeping the schema as simple as possible (star schema or snowflake vs. third normal form OLTP) is generally the way to go as your database can answer "reporting" questions more quickly.
5. Some would consider these pain points: Understanding Functional Differences Between OLAP and Relational Data Source Connections | Tableau Software.I prefer not to use cubes as a data source if the original star schema that feeds the cube is available instead.