We are working on a Collector application where users will add attribute information in related tables. We are using a feature service (AGS 10.3) that is consumed by an ArcGIS Collector app in ArcGIS Online. We don't want the feature class to be editable (that contains the geometry), but we do want the related tables of that feature class to be editable. Is this possible?
I have had success at completing this via Android (but not iOS). If you publish your feature service with editing enabled on the main feature class, as well as the related tables, then add all your feature services into AGO, you can configure your map to disable edits on the main locational layer, and enable editing on the related tables. This has worked great for me in Android devices and I have a support ticket in for iOS as it doesn't appear to work the same. I hope this helps.