They re-booked you onto a different flight at your request. Updated your ticket accordingly and charged you the correct amount for those actions, or at least I don't see anything you've written as challenging that?

I really don't see how you can argue that the change fee and fare difference was wrongly collected just because eventually the flight didn't actually operate.

You might as well argue they were wrong to sell you a return flight in the first place as the return flight didn't happen ...