Maybe he spoke with fluency because he had practiced saying those words - over and over until he was able to speak those particular English words with fluency.
An excellent point! It seems likely that these hijackings were very carefully rehearsed. Atta, or whoever was in charge, must have made sure that all the hijacking crews had a meticulous plan that would prevent any screwups. The biggest problem with Flight 93 was that it left 45 minutes late, so the passengers who called home found out about the WTC and knew what was going to happen to them unless they did something, and they had time to formulate a plan and carry it out. Also, Jarrah and company could not have known that Flight 93's passengers included so many former athletes that would easily be able to overpower them.
The letter could have been mis-addressed because Jarrah was addressing it from memory, or writing it in English or German, with which he was not as familiar, and misplaced a number or misspelled a street name. Or, he could have left it for someone else to mail, along with the anthrax letters.