Toby, I am not having a particular go, more that "apparent" safety is very prevalent across social media. The current "knowledge" is that reclining seats should not be used with harness. I too have raced for many years.
The point I wanted to make, so that everyone is aware, not poke at you, is that harnesses need to be fitted correctly if they are to be of any use. Remember, you are responsible for your passenger. They are not driving. You bin it, he gets a rib rammed up into his hung or heart, that is on you, he is not driving. ARDS instructors would get serious grief over not sorting that properly.
Secondly, from your video, which may or may not be the case, your shoulders look to be well below the harness hoops on the seat back. In a shunt the primary load is not going to be taken by your shoulders and you being held back, it is going to be your mass going forward pulling on the bottom of the seat loops which is going to want to pull the seat back down. The reason race car seats rarely move is because the belts and seats are positioned correctly so that the drivers body is taking the load not the seat. Recliner or fixed FIA bucket, those 4 little M8 screws into those little inserts in the GRP or Kevlar seat would break regardless if the seat and not the drivers body was taking the load. You may have seen at Autosport this year the latest designs which require the seat back to also be fixed to the roll cage.
Pretensioner's are indeed to tighten the belt, but this is also to take the slack out of belts. They are then backed up these day with load limiting tear plates, pioneered by Renault, to allow some give and movement to limit the energy transferred into the body on impact and these are backed up by the Airbag to cushion the head. Strap yourself in tight so that none of this occurs and yes, you arrive at exactly the reason why HANS was designed.
Yes you do only live once. Having driven many, many laps of the Ring, I have also been on the scene of many accidents there. I have lifted bikes off riders screaming with pain, parked my car so as to prevent other running them over, turned car engines off crashed cars because the drivers were unconscious and held many hands until the ambulance arrived. I have seen cars drop oil and following drivers have to be cut from their cars and flown to hospital with very serious injuries. You do only live once, but it can change very fast in the Eifel.
Be safe and look after the others that are trusting you.