Golfwise, you say that the average punter mounts a cage to the floor and rear parcel shelf. This was the case a few years ago but common practice these days is to mount them on a box section off the inner sill and to the rear wheel arch, both very strong mountint points. I didnt mention bolting to the B pillar as this is also the norm these days.
Where did the comment of cages on karts stem from? As karts dont have moving suspension they rely on chassis flex to help generate grip, there are various bars, seat stays etc. on a kart that have adjustment to vary how much shassis flex you get at any time. Remove this flex on a kart and it would be totally useless. But the principles for karts are so far removed from the principles for a car with moving suspension that they should not be compared. I am not wanting to start an argument with you, just stating some facts.
Having been playing with race cars as long as you have you must have gained
heaps of experience.