I don't understand what you mean by "attribute values that are coming from an internal table query are returned". Could be more specific about this? Are you using PI OData against your database or against NuGreen one which was used in the vCampus lab?
Are you getting some error or just an empty result?
Normally, PI OData impersonates the user making the query so if you see the data e.g. using the PI System Explorer, it should work. I suggest you turn on the tracing (see User Guide for details) and/or install Fiddler to see what queries Excel executes against the service.
Assuming 1) the issue is that you don't get data from PI Point Attributes and 2) you have a trust to the PI server from the node where you run your webservice, i am clueless.
That you do get data from the Internal table, indeed seems to prove that this is an issue when making the 'hop' to the PI Server.
Without Trusts, i agree this could be an Authentication issue (multi-hop & no Kerberos?).