Is the inReach Sent Confirmation - just unit sending or also satellite receive confirmation?

Any message exchange with the Iridium network is acknowledged. Can't vouch for the chirp. But if the unit says the message was sent, then the Iridium network acknowledged it. But this is only the satellite overhead. Iridium relays through a mesh network of satellites until it reaches one over a ground station. Then it goes to the ground station, and on to the Garmin servers. From there, it'll go to an email provider or to an SMS gateway. 

Bottom line: Lots of moving parts. The acknowledgement is not end to end. The only thing the "sent status" on the unit is good for is tell you that you don't need to continue to look for a good sky view in order to do the send.