I have done a MR with DL with a same-plane turnaround in SEA and just checked in at the gate when I arrived.
Same with three back-to-back MR flights into and out of YYZ earlier this month - just checked in once at my destination. One time we were over 6 hours late and I still could check in once I arrived.
Great that you were able to check in online but not sure that is necessary 99% of the time. I certainly would not pay for the over-priced wifi just to check in (but then I am very frugal).
Ya, I just wanted an excuse to get the wifi