OP - Just file the DOT complaint. If you had done this after one attempt at making a claim with DL, you would have either a replacement bag or a check for a new one already.

But, when you file your DOT complaint --- calm down before you file, keep it short, stick to the facts and have someone proofread it. None of the stuff about whether your bag fits the '175 matters. All that matters is that you checked the bag and it was damaged while under DL's care.

DOT will log the complaint, forward it to DL for a response and someone at DL will process it.