[Montellese]

I started looking at it but due to other priorities didn't make much progress.  Aside from the code your provided is there some place where there is more documentation on the integration ?   Maybe because I don't run MediaImport I am missing how integration with the Mezzmo addon would work since it already imports all Mezzmo data into the Kodi database and plays nice with other data so it peacefully coexists with anything else, including any data which MediaImport adds to the database from other sources.  It verifies separation of the data by keying off of the path table entries.  The Mezzmo addon tracks its path table entries and only touches database entries which join to that data. 

Media import would take care of this for you. You wouldn't have to write directly to the Kodi database (which is not well received when it comes to support) and you wouldn't have to take care of separating your media data from the rest of Kodi's database / library. The idea is that the add-on provides a way to connect to the media provider (Mezzmo server) through some custom settings and then the user can specify which media types (movies, tvshows, musicvideos) to import. Then Kodi calls the add-on to provide all media items of a specific media type. The add-on retrieves the items and passes them back to Kodi which takes care of synchronizing them into the database. Obviously there are some advanced concepts like partial synchronization etc. but this should give you a general idea.

I started picking around looking at the JSON RPC documentation but most of it seems to be for controlling Kodi interface functions and I can't find much on using JSON RPC to add, change delete data in the Kodi database.

Media import has nothing to do with JSON-RPC.

Lastly, I thought MediaImport will find and discovers uPNP servers so I would think it will find a Mezzmo server and use its normal uPNP import methods ?  For the Mezzmo addon there are a few special calls in Mezzmo which allows the addon to leverage a bulk import from Mezzmo vs. walking the database tree but generic uPNP browsing of the Mezzmo data works with any uPNP client.

I haven't used the UPnP import for a while but the main problem with non-Kodi UPnP servers is that they often don't implement the Search functionality which is necessary to retrieve all media items of a specific media type. I had to add this functionality to Kodi when I started with media import because it didn't support it either. Furthermore UPnP is limited in the type of metadata it can provide.