There's one big problem: OP was offered to be rebooked on another airline, which would have meant that (s)he would have made the (rebooked) connection. Per post #6 OP declined this rebooking. In my opinion this essentially absolves DL from any further rebooking for this multiple PNR trip, since after all DL offered a very reasonable solution to the problem that was declined...

OP: why did you want to be booked back on your original flight? If it was just because of your hotel booking, the lesson here is not to book hotels because of flight delays unless it is 100% sure that you will need one.