Martin probably cannot comment on this (any more) because of his agreement.
It looks to me the roots of these problems stems into original design. It is understood the first plan was to go singlespeed. This is also when motor design was started and main parameters decided on - size, power, rough dimensions.
Sometime after that came Elon with twospeed gearbox demand and they had to figure out how to do it. Motor developement had probably went to far already to restart its design to give it a more managable (lower) momentum so it was transmission which had to adopt. Transmission guys do not have much experience with electric motors or high momentum engines. They know much more about ICE specifics, so they probably just used their knowledge about highrev ICE engines and maybe strengthen a few places where they knew higher momentum could be a problem. Having few experiences gives high chance of suprises and transmission turned out not be up to the task of handling all that momentum and high revs.
I guess it is possible that future will show that there are two right ways:
1. single speed
No transmission, no problem with momentum
2. fluid coupling or wet clutch
Those do not engage suddenly but slowly increase the grip on new gear ratio. The oil is used to transfer torque and absorb surplus rotational energy as it heats up. A racing version would add oil cooler
Edit:
BUT: Why not just add a middle gear with some in-betweeen ratio? If revs need to drop from 200/s to only 150/s that drastically lowers the forces involved. Any idea why TM didn't go this way?