I think you do not follow my instructions or you do not know how to make a copy of a database by backup and restore. I just tested it and it works for me. It doesn't matter about exec permissions since the writes happen on the copy it does not update the database in the read connection string and no errors happen for me.
1. Content must be completely created using only MSSQLConnectionString
2. Make a backup of the database configured in this connection string.
3. Create a new db with a different name, right click it and choose Tasks > Restore > Database
4. Check the radio button "From Device" browse to the backup created in step 2, then check the box once it shows in the list of devices
5. Click Options then check Overwrite Existing DB
6. In the section "Restore the database files as" you must browse and select the files for the db created in step 3
7. After restore is complete set the MSSQLWriteConnectionString to point to the new db
It works for me. If I edit site content it does not get updated in the read connection string db so no edits appear on the site.