@RetiredF4
What is the reason behind to unlock a safety feature that early in flight?
Good find that patent!
About the unlocking, to my understanding:
The earlier in flight that you can allow unlocking - provided already having reached safe margin to uncommanded feathering through the transsonic regime - the earlier you can detect an unlocking failure, the earlier you can cut-off the rocket motor.
The earlier you stop pushing energy into the vehicle - which has been found to be unable to perform the nominal high drag reentry - the better the chances for surviving a non-nominal low drag re-entry.
In addition to that I would expect after engine cut-off the oxidiser to be dumped overboard thus reducing weight, to further improve re-entry margins.
but does that apply to a testflight where neither achieved Mach nor achieved altitude would be high enough that usage of said system is critical?
My post perhaps superfluous, reading the later portion of yours.
Hmmm, ...
I guess that also in a lower energy testflight an earlier than engine cut-off verification would add to re-entry safety margin as well.
Other than that, to test the procedure as such as closely as possible to the procedure for nominal energy flight.
Or, ... perhaps to more precisely quantify the first safety margin, i.e. to step-wise determine in subsequent tests how soon you can safely permit unlocking.
Admittedly more speculatively all that ...