On the other hand I have never been able to check in for the return until the outbound is complete, including yesterday (on my final MR of the year). I know folks say they do it all the time but neither OLCI nor airside Mr. Chickens will allow me to do it.
That said, you can check in at the podium, i.e., get off the plane and get in line at the same gate (or any airside UA podium that's staffed but not busy).
That's strange. Are you sure you're looking at the right spot when you OLCI? You have to click a small link at the bottom of the outbound itin that says something like "check in for other flights."
About checking in at the podium at your destination, I would most certainly
not recommend doing this. On a fast turnaround, you could miss the checkin cutoff and be booted from the flight entirely. Check in at the podium of your
outbound flight, before you board. This should be no problem.