Its outlandish to ask ford for any help being that you had a tune of any kind let alone a 93 tune
Not sure why your mechanic would suggest so strongly that a tune couldn't contribute to the failure....was it the cause? Who knows...maybe and maybe not....more boost and more spark can very easily go boom
Everyone needs to know the risk when you flash the ecu...ford has no idea how the tuner set up the tune so even if under warranty it would be perfectly fair for ford not to touch the car...thats why a trusted tuner is a must and even that is no guarantee....unlike exotic cars, not every connecting rod and piston is X-rayed in a taurus...some parts are tested so defects slip through
All that said im sorry for your loss