Only thing I could think of is that you would have to create two views. The first view allows adds or edits to only the table in question with all other tables and the layer removed from the view. The second view only allows edits and you will have access to the layer and all tables. You could potentially use a link to open a new table record in the "add only" table.
How are they finding and accessing the related table records? Are they clicking on the feature, then using the chain button in Field Maps to find related records they need to edit? Are they finding the related records in some other way? (Such as scanning a barcode or something along those lines? Or are you already using URL parameters and links?)
This would definitely take some testing regardless, and "finding" the related records could be tricky. Probably also some tricks you can do with the pop-up, required fields, or something along those lines to limit the ability of them to only updating an existing record, as well.
I don't think this is impossible, but it definitely takes a workaround. It would be nice to be able to set add/update/view at the layer or table level, and I would imagine that idea might already exist somewhere in the forums here.
Hopefully, someone already has an example of some of these options above that they can share for you. The two-view option would also be a bit tricky when working offline - due to how offline databases for each view are separate disconnected databases.
I hope this helps.
Sincerely,
Mitchell Grafstein, Horticultural Inspector 1, NYS Dept. of Agriculture and Markets
