Hi,
1) The problem happen when it is viewed in mobile view (please use developer tool and set view as iPhone or another phone view)

However, I have tried the suggestion which you have posted in other thread – https://www.wpbeginner.com/plugins/how-to-fix-the-mixed-content-error-in-wordpress-step-by-step/

Edit: The secure lock is not back yet.
the secure lock is back, and upon checking the network tab in the developer tools, the dummy image is now shown as “failed to load”.

How to remove this entirely, instead of it showing as an error (failed to load)?
See attached.
You need to view as a mobile view. Desktop view is fine.