I've used Mister Easy Check-In for award flights, and confirmed upgraded flights using both North American Confirmed HK49s or 500 milers that had cleared in advance of check-in.

No problem for me either.

I have never used Mister Easy Check-In on a flight I had not cleared the upgrade yet, and needed to be put on the DM list at the airport. I'm assuming that in this case, it would ask you if you still wanted to standby for the upgrade, put you on the DM list, and issue you your Economy boarding pass?

I've never received the SSSS even once from any source (human or machine).