Shamim's Plugins

You can use fep_filter_rest_users_args hook.
As $args is supplied to get_users function you can modify any parameters there. you can get supported parameters in https://developer.wordpress.org/reference/classes/wp_user_query/prepare_query/

It is added but not yet released. If you know how to change from git commit you can see https://github.com/shamim2883/front-end-pm/commit/25abae8891de1707324e54575eacbc3551a6eeca

As for announcements there might me many users email which most of the host do not allow to send at a time. So this plugin send announcement emails using cron at a set interval. You can set that interval in Front End PM PRO > Settings > Emails
Please check following
1. Cron if working in your website.
2. Change email interval in Front End PM PRO > Settings > Emails and wait that time

Let me know.

This may be possible but there are some cases where a defined page is needed as Front End PM Page, like when sending emails, using cron or email piping.
If you know how to use wp hooks there is easy way to change the page url using fep_query_url_filter or fep_query_url_without_esc_filter hooks.

You can go to Dashboard > Front End PM PRO > Settings > Emails and empty email subjects, That will do the trick.
Let me know.

Please try following code

add_filter( 'fep_message_query_args', function( $args, $user_id ) {
	if( ! empty( $args['s'] ) && ( $user = get_user_by( 'login', trim( $args['s'] ) ) ) ) {
		$args['participant_query'][] = array(
			'mgs_participant' => $user->ID,
		);
		unset( $args['s'] );
	}
	return $args;
}, 10, 2 );

Which plugin you are using for subscriber newsletter? If you use contact form 7 you can follow https://www.shamimsplugins.com/docs/advanced-nocaptcha-recaptcha/getting-started-advanced-nocaptcha-recaptcha/implement-in-contact-form-7/

Please go to Dashboard > Front End PM > Settings > Recipients
and set Max Recipients to 0 (Zero) (This will restrict users to send message to other users)
and checked “Can send to admin” (This will allow users to send message to this selected admins only)