Other mistake: In the second scene, where we see the colonel, he is looking through a door to another room. After the door closes, we see that he was looking down, into the room below. But, as we saw in the previous shot, his tie was not dangling downwards as it should have been. It wasn't a Gravity Shift, as he never entered that Cube that the camera was in.
Continuity mistake: Just after Jerry writes the number 4 on the room where he meets Kate and Sasha, they cut to a far shot as he and Kate converse. The spot where he wrote is visible and the marking is gone.
Visible crew/equipment: Just after Mrs. Paley joins the others, the door closes and a boom mic can be seen in the reflection of the door, lowering down a bit.
Continuity mistake: When we first see the colonel, he grabs the suitcase to examine it. In the far shot, you can see that the handcuffs are on the far side of the handle, near his body. The next shot is a closeup of the suitcase, and the handcuffs are now on the opposite side of the handle, away from his body.
Continuity mistake: When Kate turns to see all the numbers appear on the silver frame around the portal, although they are meant to be in the same position and handwriting as when they were drawn, much of the writing is not. (01:17:25)
Continuity mistake: When Jerry scratches a '5' with his watch onto the wall, in a wide shot he starts to scratch the 5, then in the closeup of him scratching it onto the wall, much more of the 5 has been completed between shots. (00:17:35)
Revealing mistake: As Sasha stands up, saying she feels something is wrong, to Kate (in the Razorsphere room), you can see smudges and scratches, revealing a protective glass screen between her and the camera. (00:51:30)
Chosen answer: I'm inclined to think it was longer than a month because of the lack of proper needed care for such an injury would prolong the healing process. But he only had a handful of souvenirs on him, which means he probably stopped collecting after a while. I concur with your cannibalism theory, there's no other way he would have survived. However that doesn't explain what he could have done for water. This theory of Simon spending possibly months in the cube also brings up the question: If the cube breaks down at 6:06:59, then how is it possible to live in there for months? Well, as we saw, certain rooms have slower and faster rates of time. So, he could have spent all that time in a room with a slow time rate waiting for people to wander into it and feed off of them.