Hi Dennis, lots of questions, so I'll address each one...
Are there attributes from the non-spatial table required to be viewed while editing?
Yes... hypothetical scenario. I have someone in the field that is recording the well level and are using the GIS/GPS capability of the Spatial map. They need to record the pipe diameter and make some notes in the well log table. So I have a Join between the Well ID of the GeoDatabase to the SQL server that houses all of the Well info that is also served to unrelated Web applications, devices etc... In this case I need to update the SQL server data but can't since the Join is not editable
Have you tried doing the join and serve it as a regular map service?
Yes, works just fine.... providing the table/field follow the ESRI requirements (Field name, length etc...)
I would think a feature class with a join should work as a regular non-editable map service. I'm thinking you could wire up two map services that are both included in your Online Map, one for viewing the updated information from the join and the other being the editable feature service.
Yes, but again the Editable layer in my example, the Point locations of the wells in the GeoDatabase, I can not use ArcGIS Feature Access to connect/join that spatial layer to the SQL server in order to edit.
What is the origin of the data in the SDE feature class? Is it derived from a non-spatial database also or is it maintained strictly via a Geodatabase edit session?
Well the GeoDB has to at least have the Lat/Long and an ID, so you could say it was derived from non-spatial data. Nearly all of the data join and edits I would be using is a simple point class.. Using that layer to identify and find a location that then links to another database to edit data from.
If maintained only via Geodatabase edits then consider building an SDE View. In the SDE View try to join the non-spatial table to the feature class. If that's successful then you should see it as another feature class but I'm not sure if you will be able to edit that. Something to try anyhow.
Tried that, the internal ESRI "elves" saw that and didn't like it I got errors like unknown DBMS error or similar even though I was able to open it up in studio w/o any issues.
The Add XY method should work and carry thru to ArcGIS Online as a "read only" regular map service. At least it did pre SP1 for 10.1. This idea only applies though if all of your attributes are being derived from the non-spatial data source.
This suggestion/idea/comment, a bit unclear to me
Thanks
Jason