My non techie question is on capabilities to develop an external application that retrieves artifact details from RDNG. 

Think of RDNG as a repository of requirements information that needs to be made available to an internal user community to view simple artifact information without signing in to RDNG, when supporting a client question or investigating application behavior etc.

I did some searching and found this ref in a previous question 177168:

if you want to use Java, you should have a look at the Eclipse Lyo project which implements the OSLC RM specification (Lyo Tutorial). But, Lyo (more precisely OSLC) does not provide full accessibility concerning DNG (see my question). 

If you only need to get artifacts from the database, update links and write the changes back to the database, Lyo's DNG support could be fine for you.
If you need full accessibility, you should think about using the DNG Javascript API (which provides more and more functions) - check the links in Roberts post.

Best regards
-- Thomas Noack

Given my stated requirement above, is this the net and correct?  Any other options?

- DNG JavaScript API is for RM "client extensions" (user experience gadgets in the mini dashboard) and not for integration with an external tool. 

- Eclipse Lyo implements the OSLC RM specification and meets the basic need for use in an external tool to retrieve specific artifact data (attributes).

Thanks for those that read this far and anyone with an educated opinion!