To reverse add following code in your child theme’s functions.php

add_filter( 'fep_filter_get_message_with_replies', function( $args ){
    $args['order'] = 'DESC';
    return $args;
});

To show reply form at top you will need to override view-message.php template. Instruction in https://www.shamimsplugins.com/docs/front-end-pm/customization/change-templates-2/

OR you can add a button to jump to reply form. add <a href="#fep-content-single-reply-form">Reply</a> where you want to show that button.