First; I'm sorry for your loss. :-(
Hard to say for sure, but are there black spots in the second picture? If so, this would be the second occurrence of such in your tank that we've seen, so that's a pretty big indicator that you have an issue in that area. Again; might not actually be the case as the picture makes it hard to tell if that's what it is.
In regards to the temp; do you know why the temp dropped? Did the heater fail? If so, did you inspect it to determine if it could have resulted in anything leaking into the tank?
The drop in temp likely caused a stress event, which would have weakened the fish's immune system. This would allow any latent diseases or parasites an open door to attack and overcome whatever immunity the fish had developed while in the medicated tanks of the LFS.
Edit - I think I misunderstood; was the incoming water 73F? Or did the tank drop to 73F? Or both...? Basically; if the tank had a significant drop in temp, then the above could be the case. :-|