The aircraft was a B757-200, and all of TUI Airways' fleet are a Y221 configuration.
I am not sure that is 100% true. G-BYAW & G-BYAY are shown as having 223 seats, while G-CPEV is listed as 225 seats. (all "Y").
I agree with you re the irrelevance of the change of aircraft. That must have been known 4 or 5 hours prior to the departure from Mahon so can't have been a surprise to the airline.
Anyway, it seems TUI have fully accepted responsibility as they have paid the family a full £1,300 refund for the entire cost of their flights.