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);

I have updated above code. Please update your code with this one.
CSS in theme depended. It is better if you use developer option of your browser to live change your css and determine what you need and add accordingly.

I have updated above code. Please use this code.
Then add following CSS in your theme’s style.css

.fep-column-cus_count {
    width: 50px;
}
.fep-column-cus_date {
    width: 100px;
}