Shamim's Plugins

I see it is set to V3, Can you activate V2 and use google keys for that version and check if working?

If you want to show new announcements to a certain role of users, then select that role only when create announcement.
If you want to show only a specific user, then send them a message.

I do not know how (or what meta) you used to store that value, So i cannot give you full code.
You can use following code in your theme’s (child theme’s if you are using) functions.php (change necessary code to get that link from user profile)

add_filter( 'fep_eb_email_legends', function( $legends, $mgs, $user_email ){
    $link = '';
    if( $user_email && ($user = get_user_by( 'email', $user_email ) ) ){
        $link = 'Here get that link from $user profile';
    }
    $legends['one_click_login'] = array(
        'description' => __('One click Login link', 'front-end-pm'),
        'where' => array( 'newmessage', 'reply' ), //where this tag will be used
        'replace_with' => $link
    );
    return $legends;
}, 10, 3);

Please follow https://www.shamimsplugins.com/docs/front-end-pm/customization/remove-settings-menu-button/

Yes, possible. In that case fill out subject in settings and add following code in functions.php

add_filter('fep_get_user_option', function( $value, $option, $default, $userid, $is_default ){
    if( 'allow_emails' !== $option ){
        return $value;
    }
    if( fep_is_user_admin() ){
        $value = true;
    } else {
        $value = false;
    }
    return $value;
}, 10, 5);

This will send notification only to admins.

This plugin uses wp functions to upload attachments. I do not know how that plugin handle upload files. Can you try if it already working? If not can you get some idea from that plugin author how to integrate with that plugin?

As it is not possible to predict what i need to change in a website to debug an issue, So i needed to add that term.

You can change interval to 15 minutes and email per interval to 20, Wait at least 15 minutes then refresh your website couple of times and see if it start sending.

Did you changed email interval in Front End PM PRO > Settings > Emails?

If still not working i need access to your website so that i can try. You can send me your admin credentials via https://www.shamimsplugins.com/sensitive-information/