The 'IF something comes of it" is the point though. He doesn't know that anything will yet notified them when he didn't have to and knew that this would be misunderstood. If something DID come of it then he would have to notify congress of course, but as he states in the memo the FBI does not normally inform Congress on ongoing investigations.
yes but it looks way worse if it leaks out (which it almost certainly would). If he gets ahead of it and explains what's going on which he did in the letter then that's as far as it goes until they finish. Anything else would look like a cover up or incompetence (depending on your view)