that must not be armed before Mach 1.4
I do not believe that was what the NTSB said... but more along the lines of "would be armed at M1.4" i.e. that was what the SOP / Checklist / Test Card etc. said.
One needs to understand the rationale for the "locking" lever.. what "hazard" was it trying to mitigate? What eventually might have happened? Or was the greater hazard considered that of "feathering" whilst under WK2? Or might one need the feathering quickly in an emergency hence it was armed well in advance of when it would normally be needed.
As others have said, under 60Klb of rocket thrust, the acceleration from M1 to M1.4 I doubt would be many seconds, and guess feathering equally catastrophic at either on the ascent phase?
The NTSB updates / docket system will be interesting...