we know that the airframe design was intended to have relatively benign stall characteristics -
Yes.
meaning that at the stall boundary there would be a vibration aspect
Yes.
followed by a relatively stable "mushing" descent profile.
No.
Benign stall characteristics regarding the certification testing mean there is pronounced stall buffet, there is natural tendency to pitch down at stall (it doesn't imply it has be such that it can not be overriden by application of controls), there is no violent roll associated with airflow separation and aeroplane can be unstalled using conventional technique. If such characteristics cannot be achieved by natural means, it is allowed to use synthetic stall warning and preventing devices (shaker and pusher). There is no requirement for aeroplane to gently mush if crew insists on keeping AoA high by pulling. As a side note: in West Carribean 708 disaster, captain was so obsessed with keeping the nose up he manual wounded the trim to full nose up position. So much about the evils of autotrim.
Flight testing on modern types involved an unprecedented ability to capture data that could be fed back into the computers, allowing for extrapolation of that data to determine airframe behaviour beyond what would be considered safe in terms of a physical test.
Yes.
Older types were tested up to and beyond the stall boundary, but there was no way to capture the physical data in the same manner, and thus no way to feed that into simulated behaviour.
Correct but could be misleading. New types are tested up to and beyond stall boundary too. For certification they are just pushed beyond the lift limit into stall and promptly recovered, there is no requirement (or reason) to pull them into extreme alpha.