code order is the enemy here.
in the Universal Email behaivor, you have set the to address to use the eCartCheckoutForm_email session variable.
the universal email code is at lines 24 - 111
The code to set the session is after that at lines 149 - 154.
this means that the session does not exist yet when the email is trying to send, so the to address gets sent to empty value.
you need to either set the to address to use the email POST, or move the UE code so it is after the set session code.