Then, at about line 600 where the comment field cell is located we change
<td class="main"><?php echo tep_draw_textarea_field('comments', 'soft', '60', '5'); ?></td>
to
<td class="main"><?php echo tep_draw_textarea_field('comments', 'soft', '60', '5', mce_editable="true"); ?></td>
This should make the field editable. The remainder of the field handling does not seem to me to need any change.
Now all that is left, is to set the email to HTML type. Forcing this through the configuration values may be all that is needed here.