1) Yes, you can reference the DLL containing the business objects within your Web Site / Web Application. Just add it like a regular reference.
2) We have help documentation on using the templates for Web Apps. The main one you need to look for is the ApplicationBasePage template. There is information in it that details how you can create a property on it for each of your business objects. These values are then read by the ApplicationBasePage and stored/retrieved from session variables to maintain the state of the business objects across multiple pages. Beyond that, the webforms programming is much like any other web forms programming except you use business objects to retrieve your data rather than raw ADO.NET.
3) No issues with using DevExpress stuff. It should work just fine.