There are a few questions that still remain for me. In the scene in Natalie's living room, where Leonard flashes to a syringe of insulin, how much insulin is in that syringe? To my eye, it looks like a whole lot. Even after pausing and zooming, there aren't any numbers visible. This makes me wonder.
There are a number of scenes where Leonard's memory works when it probably shouldn't have. For example, when Natalie is trying to rip the picture of Dodd, Leonard tells her that she needs to burn it. How would he remember that tearing wouldn't work?
I think that Leonard's condition is real, but not because he can't make new memories. There are things that he doesn't want to remember. So he forgets. Some things slip through, though.
The story of Sammy Jankis would seem to be Leonard's story. But since he cannot "remember" it, as it happened after the incident, he projects it onto someone else.
Mrs. Jankis would get Sammy to hide food around the house, and then stop feeding him to see if hunger would cause him to remember where things were hidden. Maybe Sammy/Leonard's memory slipped (or in this case, didn't slip) about some of that spousal abuse. So I ask again, how much insulin was in the syringe that Leonard remembered?
Did Leonard kill his wife in the way that he relates it in the story about Sammy? Or was it more purposeful?