OP, sorry to hear your story, but seems like you did all your could. The only other thing I would have tried is calling the PM line to see if someone else could have helped you better. AA/UA options in ORD make sense, I thought DL could do that but sounds like you had an unfriendly agent.

DL often has issues with delayed planes coming in and then blaming the delays on things like weather (say if first delayed for mechanical, then further delayed by weather DL will call the delay "Weather").

Write back to DL and see if TTU does anything.