I can’t make much sense of most of that code. Does function bp_core_signup_show_user_form take the input from the form? Or does it only output the form? I guess the latter.
I think the code that actually processes the input is in bp_xprofile/bp-xprofile-classes.php, Class BP_XProfile_ProfileData etc. Or bp-xprofile-signup.php?
Back to the other thread to see if it gets me any further…