1. NAc Agent is an actual software installed on PC usually used on corporate computer while NAC web agent is temporal web-based agent running on ActiveX (I believe) and user does not require software installed, usually used for guest
2. No, NAC agent can run on any Windows/Mac regardless of domain membership
3. Yes, it is just a piece of software to install. What's more important is assigning propoer authorization profile to users.
4. Please clarify the question
5. Please explain what you are trying to achieve
6. All ISE hardware appliance supports IPEP. Please see ISE install guide