Hi!
1. Well, in simple words, ER is a way of representing models, you can have Relational, Dimensional o Conceptual models in ER diagrams. If you mean Relational by ER, the main difference is that Relational represents Business Rules and Dimensional represents Navigation Rules (how information flows versus how information is being shown)
2. Usually Relational models are 3NF and Dimensional could be under 3NF or another modeling technique as Stars or Snowflakes
3. It's a good practice for EDW to get constructed as a 3NF Relational Data Model, and using an ER Diagram for showing users and IT people its value. 3NF delivers more value and an application neutral view of the information.
4. A Datamart is a specific subject oriented part of the warehouse and is usually built as a Dimensional model, because it refers more to navigating the infromation.
5. Yes, they definitively do
Hope this helps!
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