While taking seats out today to deal with wet carpet from clogged sunroof drains, I got distracted and didn't properly set my ratchet when removing the rear passenger side lower seatbelt bolt. I thought it was rather tight and then I got some movement but noted it seemed to spin in place. I then realized I was turning to tighten the bolt instead of loosening.

Got the bolt out and looked like the threads on it were knackered. Before reassembling everything I tried running the bolt through a M10 1.5 die. As I did I noted that the bolt was fine but it had stripped some of the thread off the nut which had wrapped around the bolt. Cleaned that off and ran a tap through the hole hoping to fix the damaged threads. Bolt feeds in fine but I am unable to tighten to spec. Bolt gets snug then starts to loosen off. Appears that the nut lost too much material when I overtightened and now doesn't have the gripping power.

What are my options here? As I am dealing with a seatbelt, it needs to be secure. Was thinking of trying to tap the hole to 1.25 pitch and do the same to the pitch on the bolt but if there isn't enough material then I have ruined both the nut and the bolt. I have never used a helicoil but would that type of repair be an option. The nut is on the other side of the metal and I suspect welded to the skin. I don't know what I am working with and if it would take being drilled out larger to accommodate the coil repair.