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Since 'deleting after payment so long as the original debt is at least 2 years old' is Midland's publically stated policy -- stop sending PFD letters. Instead, offer a settlement amount without mention of tradeline deletion in return. If dealing with them over the phone is problematic, then use their website to look up your account and see if any offers exist for your account: https://www.midlandcredit.com/help-center/credit-reporting/.
Also understand that if the debt is still with the Statute of Limitiations for debt collection in your state, you hold very little leverage, if any, when negotiating a settlement amount -- collection agencies, in general, are much more willing to settle for less when they are no longer able to sue for the amount owed. The Midland collection that's now with the law firm may require payment in full at this point as they've now incurred attorney fees.
Yes, begin sending Goodwill letters to Transworld. Goodwill's are meant to be personal pleas to the creditor/collection agency. Don't copy someone else's letter- write your own and explain what caused the original debt, what you've done to better your debt management habits, and how you plan to maintain those good habits in the future; explain what having the tradeline removed would mean for you and your credit. It should be unique and genuine -- your goal is to pull on the heartstings of the person reading the letter and make them want to give you a second chance at good credit.