Regarding the problem with aspNetCompatibilityEnabled, this has been fixed as of version 5.1.1, available tomorrow. In prior versions the DevForce services would not work when this flag was enabled.
We have no recommendation about non-database entities. You could write a web/WCF service and consume them directly, or consume them as an addition to your domain model (although web service backed objects in DevForce Universal/SL have some limitations). Version 5.1.1 has also introduced support for POCO objects - so you could also write your own very simple provider and then consume these objects via the DevForce EntityManager. The release notes will have more information on this.