I think it is unfortunate that Dr. Mueller was not invited to speak, and my understanding that this was essentially reciprocity for David Swartling not being invited to speak at the LCMS convention last year.
On the other hand, very few ecumenical guests here were invited to speak or bring greetings: the AMEZion bishop, the ELCIC bishop, and then a couple of international guests.
But on still another hand, it is telling, I think, that there was also no official greeting, even by video, from the Roman Catholic Church--and you will recall that the video greeting from them last year included some criticism about the sexuality (at that time) proposals, which, while gentler than Dr. Kieschnick's remarks, nonetheless caused considerable consternation. The back story there was that the powers that be tried to axe that video greeting, and it was only strenuous objection from Don McCoid that prevented it from being dropped. So one can understand the perception that the ELCA doesn't want to hear greetings from anyone who won't be politely discreet and avoid mentioning the elephant in the room.
I think it is quite likely that Dr. Mueller's not knowing ahead of time was a miscommunication, and that is unfortunate. It is possible that it was a last-minute decision, but that is not my impression from my conversations.