i would not take no for an answer, i am a mechanic myself with out actually seeing vehicle i can obviously not be much of a help but if i had replaced a tranny on a vehicle and 2 weeks later the flex-plate broke (flywheel is only on manual tranny auto tranny has a flex-plate.) i would assume that something i had done was most likely the cause. or at very least i should have noticed it was damaged before reinstalled the tranny. i suspect that if it is the flex-plate that one of the following may be the cause, they actually did take off flex-plate and did not re tighten it properly, that they may had damaged it while taking tranny out or reinstalling it or possibly the torque converter was not installed properly or had something on it when it was installed and was trapped between flex-plate and torque convert, and was ok for a while but eventually flex plate fatigued under load and high temps that torque converter reaches.

if i were you i would try to be there when they pull tranny out so they don't have time to hide anything from you if it is there fault.

hope this help and best of luck