1. You can override viewmessage.php template to achieve what you want. Instruction in https://www.shamimsplugins.com/docs/front-end-pm-pro/customization-front-end-pm-pro/change-templates/

2.+ 3. add following code in your child theme’s functions.php

add_filter('fep_message_table_columns', function( $columns ){
	$columns['cus_count'] = 'count';
	$columns['cus_date'] = 'date'; 
	return $columns;
});

add_action( 'fep_message_table_column_content_cus_count', function(){
	$args = array(
		'post_type' => 'fep_message',
		'post_status' => 'publish',
		'post_parent' => fep_get_parent_id( get_the_ID() ),
		'posts_per_page' => -1,
		'order'=> 'ASC',
		'fields' => 'ids'
	 );
     
	 echo count( get_posts( $args ) ) + 1;
});

add_action( 'fep_message_table_column_content_cus_date', function(){
	the_time( 'M d, Y' );
});

add_action( 'fep_message_table_column_content_author', function(){
	echo fep_user_name( get_the_author_meta('ID') );
});

add_filter( 'fep_formate_date', function( $h_time, $date, $d ){
    return $date;
}, 10, 3);