1. You can pass heading to shortcode. So shortcode will be like [fep_shortcode_new_message_form heading="Anything"]
2. You can add #message_title{display:none;} in Front End PM PRO > Settings > General > Custom CSS
3. To change text add following code in your theme’s (child theme’s if any) functions.php

add_filter( 'fep_form_submit_button', function( $button, $where){
if( shortcode-newmessage == $where ){
    $button = '<button type="submit" class="fep-button" name="fep_action" value="'. esc_attr( $where ) .'">YOUR TEXT HERE</button>';
}
return $button;
}, 10, 2);