Sure. You are right. If you see notices on your screen, it means that debug mode is ok. Unfortunately, in your case, it does not help.
I intentionnally created an error in a widget. And the error message is never displayed on screen when setting WP_DEBUG to true. It is visible only on debug.log file when adding the 4 lines in wp-config.php.
I have no idea why you do not succeed to get this debug.log file in wp-content directory. That's why I suggested to setup a local install.
Maybe the php errors are logged in another file. Maybe your host can help you to know that.