1) There's nothing in DevForce Silverlight which will force your UI to be coded a certain way or be dependent upon the domain model. So, writing a view to an interface should not itself be a problem, but it's up to you to determine how you back that interface with a concrete implementation at run time. I'll add that DevForce supports service-backed entities as well as EF (database) backed entities so you could theoretically swap models based on the DevForce "data source key", but there are some limitations with service-backed entities at this time.
2) DevForce does not integrate with nHibernate and supports only Entity Framework generated models.