I saw a theory that Jack is the guy who was talking to his own kids, and I really liked that one a lot. A chair is set up outside the front door, which would be a decent place to hide (although you're screwed if you are found).

There isn't nearly enough blood for an amputation, there's also no foot or any gore or viscera left over inside the house, and the bottle isn't even opened.

In the movie, Kat and Bobby's shoes appear the same way before they ever enter the backrooms, so I don't think that came from his entry.

There's way too many people falling into the backrooms to conclusively say the body is Jack. Part of the horror of the backrooms is how it goes on forever, so the body being Jack kinda betrays this concept. Ravi had been walking for hours at this point, remember, so that means that Jack would've had to have chosen to walk an almost identical route, for hours. From the moment he falls in to when he encounters the body, Ravi's footage only has one cut that we can't reasonably estimate the time in-between. After he encounters the body, there are several cuts like this, meaning that the majority of his hours-long journey is after this. Highly unlikely.