Any doctor who knows anything about Leonard’s condition would tell you that it’s not consistent, and there are always anomalies like Leonard’s ability to remember having the condition. Maybe the movie should have told us this (I don’t recall if it does), but even if it didn’t that doesn’t constitute a major flaw and shouldn’t strain your suspension of disbelief.
And the invention of Sammy Jankis to transfer Leonard’s own guilt about what happened to his wife is also a plausible psychological phenomenon.
Next?