No 1 & 2: You can wait for the next release as we already added a space there. Or you can add a space in https://github.com/shamim2883/front-end-pm/blob/25abae8891de1707324e54575eacbc3551a6eeca/templates/view-message-content.php#L53

No 5: You can wait for the next release as we already added a “s” there. Or you can add a “s” in https://github.com/shamim2883/front-end-pm/blob/25abae8891de1707324e54575eacbc3551a6eeca/includes/class-fep-form.php#L368

(Both in this plugin in your website’s copy)

For no 3 & 4: add following code in your theme’s (child theme’s if you are using) functions.php

add_filter( 'fep_filter_read_receipt_individual', function( $mgs_read, $mgs_participant ){
    return '<div class="fep-read-receipt-individual">' . sprintf( __( 'Read by %1$s on %2$s GMT', 'front-end-pm' ), fep_user_name( $mgs_participant ), get_date_from_gmt( date( 'Y-m-d H:i:s', $mgs_read ), get_option( 'date_format' ) . ' ' . get_option( 'time_format' ) ) ) . '</div>';
}, 10, 2 );