When the"Use read-only query" option is checked I am still able to insert, update, and delete records by running queries in the Editor.
The "Use read-only query" option only affects data grids. It does not prevent users from modifying data by executing statements in the Editor.
Download and install the read-only version of Toad for Oracle, which can be found by going to https://support.quest.com/SUPPORT/index?page=downloads.
The read-only version of Toad will not allow any modifications to the database from any window.