Shamim's Plugins

It is possible but need core plugin edit. In that case if you update this plugin in future you will lose those changes.
It will be complex solution. If you still need those changes you can contact me through https://www.shamimsplugins.com/hire/

Will it query only by meta_key or meta_value as well?
Do you want it so that it add all users during group save or when a message is sent? Eg. you added this meta key in group, and later a new user is added to that meta key. So will that user receive new message to that group?

You can add add_filter( 'fep_enable_email_send', '__return_false' ); just before your function call.

It is always recommended to use hook for any customization.
I have manually activated your previous version license but it is recommended to update to latest version when possible.

Do you have your previous license? If not please contact using https://www.shamimsplugins.com/contact-us/ and i will send you.

All settings seems ok.
Can you create a new Recruitment Admin user and check if that user can send new message?

Do you have any staging website? Can you deactivate all plugins and activate default theme then try again? Then activate one by one and check when this issue re-appears?

It is possible but needs custom code. You can add a hidden field for shortcode newmessage and pass page/post id. Then you can use that id to determine what action to take

Sorry, this is beyond this plugin’s capability. You can use any 3rd party email services to see who opened/read your emails.