User blog comment:Master Sima Yi/Assassinews 05-08-'14 – Assassin's Creed: Rogue announced/@comment-5019018-20140807031511/@comment-90.198.30.169-20140808121604

Oh shoot, yeah... the 1758 onwards... I goofed there... whoops.

As for the 2 year gap. It is implied he left, pretty much, straight for England and began his travels to Turkey. Why would he get the letter and be like "I'll wait awhile". I don't know much about timings with travelling in the 18th century. In those two years he would need to travel from Boston to England, Meet with Reginald, travel with Holden from England to Istanbul and then pinpoint where Jenny is: Which Haytham says took him the two years.

17/09/57 entry: "...one that took us into the heart of the Ottoman Empire, where, for the past two years, he and I had tracked Jenny."

Which means we could see Haytham in (some of) the earlier memories and, hopefully a nice amount the latter memories. But indeed not as a permanent fixture, even within the earlier years Haytham is in Boston: he was doing this, that and the other.