re accelerometer, meant only: for testing of your scheme.
Clearly one needs One important datum:
How-many-Gs' is Max allowed before the weakest (physical) link is trashed.?
(This is apt to be a different value for various angles-of-impact
--and nobody may have spent more than $10 exploring the matter.)
Then set a "max-height above cement floor" which your device will permit, excluding your throwing the sucker at a wall, say.
The rest is pretty simple concept, testing, execution, then surveying IF the bulk is Worth the safety -vs- paying the Insurance (#s you mentioned.)
One thing is clear: I haven't seen an Otterbox, but if they are transmitting drop-shock directly to a phone
--sans obvious physics of 'crumple-rate' of substances: they are certifiably Incompetent.
Rest case.. er, case-about Cases. This stuff is fucking-Obvious, innit?
(And ya Can't 'patent' physics ... unless Patent Law IS as fubar as oft reputed.)