Marty receives the letter from the doc in 1955 after the Delorean was struck by lightning.

This Marty knows everything that will happen to the doc from 1955 (he will age 30 years, go to the future, then to the alternate present, then back to 1955... etc... and he also knows what happens to him in 1885.) Instead of risking his own life and going back to 1885, couldn't he just write another letter telling the doc to be careful with the almanac in 2015? burn it? something? He knew that the doc, who was very stubborn in 1955 about knowing the facts from the future, would read the letter eventually and change his own future ( being killed.) He could have assumed that the doc would read the 2nd letter too. Then he could have just traveled back to 1985 and we wouldn't have a 3rd movie after all. All I can say is: Thank God he didn't.