Let's not forget that the manual trim may only become immovable at or near max stabiliser travel at normal airspeeds.
It is more than a bit harsh to criticise Boeing when the pilots so mishandle (overspeed) the aircraft it is completely out of all parameters for the trim to work, and neither do they take any recovery action - ie slowing down.
Additionally, Boeing would not expect or anticipate a crew allowing the trim to get so far out of position, the rollercoaster method was a confidence demo to show that even if you did let the trim go to extremes it could never normally achieve in reality it was still recoverable. These pilots did let it get there.
If the pilots don't fly the aeroplane/procedures properly how can Boeing's procedures be expected to cope with that?
There's no certification need or requirement to have manual trim operable at full deflection above VMO - how could there be?