Marty's presence in the past affected his mother's, father's, Biff's and Doc's only (also other people's lives but that isn't relevant). When he arrives back to 1985 he discovers that Doc is still alive and was wearing a bullet proof vest. This is because as we find out, Doc read the letter, but the Doc we saw before Marty went back to 1955 really did die.
A similar thing is how Marty's family from the first 1985 seemed almost poor and just another local family and how Marty in 1955 made his father stand up for himself, if this never happened then George most likely would never have become rich and famous. So even though there was no "butterfly effect" there still is a mild one, but it was mild enough not to change the future drastically.
Doc wouldn't "already" know Marty in 1955. In the original history, Doc and Marty met by some unknown series of events, purely by happenstance (Doc perhaps taking an ad out in the paper for a lab assistant/grunt, for example; and Marty happened to answer it). Ultimately, as a result of this incidental meeting and friendship, Marty goes back in time, where he meets Doc ahead of the original sequence of events (ahead of time for Doc at least). Now as time moves forward in the new timeline, Doc knows of Marty and that they'll eventually meet; but he may still not know how. Because Doc's aware of possible repercussions of knowing about your own future, he probably wouldn't have asked Marty how they met, not would he have sought out Marty prematurely; instead patiently waited for events to occur naturally, being pleasantly surprised when it finally did happens (Such as Doc taking out an ad, only to find and recognize Marty answering the ad.)