i posted a video in the OT thread. the yt channel covers the collapse and did a full structural analysis. there was a revision of the original plans concerning the garage and a transition was removed for esthetic reasons, but they also removed some beams that were under the planter tubs for the above landscaping (trees and bushes). the live/dynamic loads from cars driving over wasnt accounted for so a bunch of supports were already at max loading.
whoever did the revision is absolutely liable.
I don't know the full history of the building.
The original engineer is only responsible for the loading conditions and building code at the time of design. If the loads (dead, live, etc.) were underestimated or the revisions during construction affected the structural integrity, then yes, he is liable.
The point is structural engineers can't be responsible for changes/modifications/additions by others after the fact if the loading condition is drastically different as long as the original design met the load requirements of the applicable building code in effect at the time.
[EDIT] The video was excellent. I don't disagree with any of his analysis but I'm not sure all the fault lies with the original designers.