I think the two bolts pictured in the center are the stock bolts.
I see you found that problem already - the captive nut on the underside has broken loose. On my car there’s no way of reaching these nuts, they’re covered by the underskin. But you were able to reach yours - did the access look like the factory designed it in, or like someone took a drill bit to the skin?
In my case, some previous owner had actually done a favor...kinda. The captured nut on all four bolts had long ago let go, so the PO had taken a hole saw and cut a small hole in the underskin. It was actually quite easy to put a ratchet w/extension on the bottom and a ratchet on the top and just release them. The PO had even saw fit to use a stainless nylock on the bottom nut. Of course, I have 8 x 1" holes in my underskin now... Also, in my case the issue wasn't a sticky seat. It was a seat that moved too freely--including most times when you applied the brake pedal. So access to the fasteners from the top was fairly simple. The problem I faced was trying to release the seat rails from the seat frame (after the seat was out of the car). Highly recommend the re-conditioning of the rails. Even tho you rarely adjust the seats, it just makes the car feel a bit less janky. Good luck!
I can take a picture of it this weekend. But it looked like it was factory? Basically it's a fairly thin "box" that has the square nut inside it. What happened on mine was the edge of the box had bent outward allowing the nut to spin. So to remove it my wife put a flathead in there and just jammed it while I loosened the bolt. I was able to fix the box by a little hammer action, but I'm highly considering a modification in case I need to remove the seats in the future... it was a bear. I don't know if I can see the front though it might be under the skin, I didn't have to deal with it since that was coming loose nicely.
On the fronts, did you check that the bolts thread back in? I don't know how the underside nut is captured (since as I mentioned above, mine aren't)...but hopefully those front nuts didn't fall off into the underskin region when you removed the front bolts.
I seem to remember that my captured nuts weren't welded in, just caught by the metal angle section that held them, as it all seemed quite loose. But I may be wrong. As others have written, I have absolutely no desire to take it apart again to find out. Twice has been enough!
Yes the fronts thread back in just fine, just haven't looked under it to see if they're set up the same way.
