You're overthinking things, the differences between any well engineered/designed seats are negligible. You are unlikely to experience the kind of forces that they are subjecting the seats to in those videos on the street (I used to work for an insurance company and have seen 100s of accidents) and if you do; most seats, OEM or not will perform in a similar manner. Even on the track its rare to be exposed to that much force. I would just get quality seats that you like and be done with it.
FWIW this was an accident I was involved in in 2011. I was stopped at a light and was rear ended by a jeep going 65+ mph and forced underneath a pickup truck hard enough to break the leaf spring mounts and separate the rearend from the truck on one side. My seat setup at the time was a rep Bride Vios fixed back with the factory 3 point seat belt. You can't tell from the pics but the OEM passenger seat broke at the base on the right side. I walked away 100% fine and wasn't even sore the next day. My passenger did suffer some whiplash and ended up doing a week or so of PT but was more or less fine as well. I definitely benefited from being in the fixed seat but likely would have been fine either way.
My current setup is an authentic Zeta II with the factory 3 point belts and a Brix 1.5 passenger seat. You can safely run the factory belts with most fixed seats, I don't recommend harnesses in street driven cars since as you mentioned above they tend to do more harm then good. My 2 cents...