Yeah, I wouldn't care too much about people. You are the priority. And fixed seat is cheap and consistent
Demon seat sucks, depending on how much it moves it is almost as bad as having a floppy shifter.
Is your seat flexing side to side? Do you have a picture of how the rails mount in your car without the seat mounted to them? I have never seen the insides of a volvo. I think I may have some side mount brackets laying in my pile of car stuff that might fix all your problems.
It looks like your bracket bolts directly to the sheet metal floor to me. If you weld a small .5" flat bar vertically in between the seat rails it would stiffen it up significantly.
If it STILL moves... I would try and get some light square tube and weld it across underneath the car so that the studs for the seat rail bolt through them. or something along those lines.