Barring the issues surrounding the current volcano situation, this is my feedback on the core question (check-in times for delayed flights).

I have asked TA about this before and been told that a delay, even if it is noted online or at the TA desk, does not impact the check-in requirements. IOW, you still have to be checked in subject to the originally scheduled departure time (both person and baggage limits in place).

From my experiences with DL/UA/WN, I would assume that is pretty much the industry stance. Keep in mind, people not hitting the original deadlines is what allows the airlines to manage their oversold and IRROPS situations, as those seats become available at the check-in deadline.

(not a DL, or even any airline employee - just posting my anecdotal experiences)