The noted damage is interesting, wonder if the broken motor mount had anything to do with putting the car in the water.
Didn't the broken engine mount issue sometimes cause the throttle to stick open, hence the recall to fit interlocking mounts?
Yes, a broken LHS motor mount *could* cause the accelerator to jam up (I had it happen to me with my dad's '63 Impala..
... but I doubt that would have been an issue with the car less than 2 yrs old? and I don't believe it had a HP engine in it... the mount could have broken as a result of hitting the water at speed...