A Simple solution
Deep down, Leonard seems to know the truth.
The lie is that Leonard killed his wife. That lie may have been started by John G. He repeats it here, even as he has repeated the lie that the real John G is dead. While this could be the truth, it is doubtful because John seems to lie at every turn and for the fact that we don't know if the tattoo itself was a lie started by Leonard.
The simple elegant truth? Leonard's wife did not die. Yes, she was attacked, but afterwards she couldn't take it...so she left. Having been left permanently is more painful that having her dead, so Leonard has retreated into this fantasy.
Leonard is constantly reminding himself that she is alive, as in the hotel room with the escort. Also, he remembers her alive in his dream near the end, that he could win her back by finally killing John G. He even relates when he is with Natalie, that he somehow knows she is gone and not coming back. Again, he doesn't say she is dead. Only on another occasion does he state that John G killed his wife, which we know is false.
John G - The odds of John G cop being assigned to a case involving John G? Million to one. It would cheapen the whole film. It is quite possible since the report has names, Like the officer being blacked out. The real officer was talking to Natalie about Leonard, not 'Teddy'. He was there to drum up a drug deal, hence, not the same person.
Leonard's tattoo only says she is gone...not dead. If he knew she was dead, then his hoax of using the memento's wouldn't work.
If Leonard had killed his wife, he wouldn't remember it, so he would have no reason to try to create some elaborate lie with Sammy. Doesn't make sense.
(reposted from a thread that got way too long and trolled by one bizarre dude.)