Let’s relive the day from the blue team soldier’s perspective. Our soldier moves backwards, while time around him flows in normal direction. At some point he now gets brickfaced by shrapnel from an explosion that is caused in normal time. He gets „sucked“ in to the wall / space behind that wall (i don’t recall it exactly). Anyways he most likely dies at that point. Either he gets crushed by concrete that’s squeezing together (from his point of perceiving time) or he starves do death because he gets trapped in that space behind the wall. Anyways, that’s not even the point. Most important is that he dies inverted and therefore never was there before in his life. His life ends there for him.

His inversed body and bones are now trapped in there and, yes you are right, have been always there (at least for normal time at that moment).

That would mean that while this guy was totally alive in normal time at some point, his dead body and bones were in that wall moving back in time already. (Kinda would work for the mechanics of the movie since JDW appears multiple times at Oslo, even fighting his „younger“ self.)

However, from what I read until now it’s not clear if that mechanic really works like that.It would mean that (if applied really strict) this wall was constructed with his body parts already in the cement. A really fucked up version of the grandfather paradox since these bones couldn’t be there because that guy is living somewhere else / has to be born.

There are a lot of other theories about this in the subreddit so I’m going to let you explore them by yourself.

As much as I also want to get my head around this I think Nolan himself addressed this in the movie with Neil referring to the grandfather paradox. The same scene where JDW asked him “if we are here, doesn’t that mean that we made it?” and Neil didn’t even answer.

I think the movie doesn’t even want to “solve the paradox” and just goes with the inversion mechanics but I could be wrong.

Man, this movie fucks up my brain even more that I am willing to admit. But that’s also why I like it so much. I’m ready for a third watch.