When ever I have had a problem with one of the Delta Connection Carriers I have written to DL and or DL and the Connection Carrier. Each time DL has handled the problem and once one of the Connection Carriers followed up with an email and written letter (ASA). Bottom line, this is DL's problem, go after DL, your ticket says "DL", it is DL's problem regardless of what they say.
I just say, don't fly DL and you wont have these problems. Of course, that does not insure you don't have the problems on other airlines... but I tend to think other airlines (or some of them...) actually care if you fly with them again.
Dreading making travel reservations... DL may be in the works for a few flights over the next few weeks.