Just to close the loop:

• I saw GRR-CLE post a delay about 20 minutes after its scheduled departure time (at 5:48 p.m., it was still posting an on-schedule departure of 5:28 p.m.).
• About 30 minutes later, when GRR-CLE was in the air with a scheduled arrival time of 7:13 p.m., I saw the 7:12 p.m. CLE-IAD post a proactive delay to 7:30 p.m.
• After the inbound had fully deplaned, the gate agent vanished for a while and chatted at the end of the jetbridge, then at about 7:40 p.m., the gate agent announced that the aircraft had been struck by lightning and was being taken out of service.
• A replacement aircraft was found, and the "7:12 p.m." flight ended up departing at 8:30 p.m.

The right person to contact for rebooking help was the United Club agent at CLE who was incredibly good.

Meanwhile, my wife was flying DTW-IAD-LHR, and her DTW-IAD was delayed an hour, destroying her connection. Hilariously, her flight was delayed because its original inbound (ORD-DTW) had diverted to CLE -- leaving CLE with an extra ERJ-145 that could be used to complete my lightning-mx'd CLE-IAD!

Unfortunately my wife wasn't advised of the delays and we didn't get to stay at CLE together; but at least we are both on the same EWR-LHR in the morning.