No problem. With this info at hand, the mahseer death is not unsettling at all, contrary to my earlier claim. But new set ups do need frequent monitoring and close watch and deaths in new setups are almost always suspicious and almost always have to do with water not being right, but not always of course.
Belly up doesn't mean a temp shock, you should have measured the water temp to arrive at this conclusion. It might have run out of oxygen or got poisoned by its own ammonia or merely too much stress from too long a shipment.
In this non-standard situation of belly up arrival, it is hard to say. I've done what you describe too in a few cases, panicked, skipped acclimation, and too lost the fishes I did it to. Perhaps if you had noted that the mahseer was still breathing, a gradual but quicker acclimation could have made a difference but this throwing darts in the dark.
