BuddyPress.org

You shouldn’t change core files.

You should be able to make your changes via template overloads.

Copy this file
bp-templates\bp-legacy\buddypress\members\single\messages\single.php
to your child theme so that the dir structure looks like this:
your-theme\buddypress\members\single\messages\single.php

Then in single.php, change
if ( bp_thread_has_messages() ) :
to

<?php  
	$args = array(
	   'order'     => 'DESC'
	);
if ( bp_thread_has_messages( $args ) ) : ?>

To change the location of the reply box, adjust the html in that same template overload.

The reply form has hidden fields that require variables only available within the bp_thread_has_messages conditional.

See https://gist.github.com/shanebp/3d04465213e99665e512

Building on this idea (which is awesome!), is there a way to have the new message post below the ‘reply-composing box’?

With this setup, new messages are posting above the ‘reply-composing box’ until the page is reloaded. I’ve tried altering the single.php to accomplish this, but it seems a function might be controlling this layout?

Thanks @shanebp and @marvin_klempert!