Hello, Sorry about my late reply.
You send envelopes to random members. You also receive envelopes from random members. This means that when you receive an envelope back is not necessary from the same member you sent to.
Each envelope has a different ID. The sent and received envelopes, all have different id's.
It is possible to have more then one account with the same address. This does not affect your incoming envelopes.
Regarding US-679, I don't know what to tell you. In this situation you can contact your exchange partner like you did already. If this does not resolve it, the envelope is considered lost in the postal system. It is possible for letters to arrive late and they still can be registered. Also, if you think that the letter was lost in the postal system by mistake, you could consider to resend another envelope.