Sorry you had such a bad experience. Delta is not responsible for Airport Security so your complaint is really with the US government. I recently almost missed a connection myself in ATL due to the extreme slowness of immigration. If you presented yourself prior to T-15 for boarding then you do indeed have a legitimate gripe with Delta regarding the loss of your seat. But if you were just after, like 14:00 before departure, they do have the right to give away your seat. In cases of a missed connection, Delta will accommodate you in the next available flight in your class of service. Generally they will not put you on another airline unless they are the cause of your delay, which in this case they were not.

As far as the baggage situation goes, it appears that they have offered you reimbursement for what they consider reasonable expenses (You haven't given an amount so I can't say if its fair or not).

You will get nothing for missed meetings, for personal accidents in the airport or delays at security.

Apart from being provided some inaccurate information regarding your baggage location, for which Delta appears to be offering compensation, I don't see that you were lied to and you certainly weren't humiliated by Delta.

You can submit a complaint online through the website, but I seriously doubt you'll get anywhere because again, its not Delta's fault you were delayed.