Just a word of advice (although you have already take it into account). I'd place a similar kind of metal plate behind that bolt through the rear vertical seat back. Even a washer gets pulled through the steel plate easily, as it's just 0.8mm. you need to spread the forces preferably into panels around it with different orientations; just like you did for the front bracket.
I regularly watch crash tests of cars in my job, and I've swept up enough bits after the bang in my lifetime.
For example; a standard M10 bolt in a matching nut it's normal strength class is not strong enough by itself. I've seen bolts being pulled out of welded-in nuts even.
The amount of force on a seatbelt reel is massive. Do not underestimate it; in a serious crash, (the one where you need the belt to keep you alive), the force on the upper belt point is about half your mass multiplied by 20, 30 or even up to 50 times if you hit a tree with just a small front overlap.
Imagine pulling on the reel with that force in a jolt; will it hold?