BuddyPress.org

If i’m reading you right all you need to do is replace bp_displayed_user_fullname() with bp_displayed_user_username() in the member-header.php file.

There’s also the function bp_member_name() that shows on the members directory loop. And then in the activity stream there is code that spits out the who when and where of each activity bp_activity_action()

These functions are mostly in each folder corresponding to that BP component so activity would be in activity/bp-activity-templatetags.php