Just for my education really, I was under the impression that the engine fuse pins (I guess Boeing-only given your comments) were also designed to shear in flight in the event of a catastrophic engine failure, mainly the onset of seizure of one or more of the shafts. Such an event, even as it developed, would start to transfer rotational energy/momentum from the shaft(s) to the engine casing and the pins were designed to let go before the twist went back up the pylon and into the spars. Nothing to do with this situation – just interested as the question of the pins has been raised!
There is no way that anybody can define catastrophic seizure loads in a design envionment (way too many what ifs). What both Airbus and Boeing do is to ensure that the critical aircraft structures (for flight) meet the min or better capability for sustaining flight loadings (gust, flutter etc.) and the engine manufacturer has to demonstrate that in a blade off test no distortion will occur in the engine mounts. Airbus and Boeing then size the in betweens the engine and the wing to sustain loads up to but not exceeding the capability of the wing.
When all is said and done the regulation does not accept engines breaking loose in a defined flight envelop but are content with the status quo as long as it does not fall off and hit somebody.