If Marty tells something to Doc in 1955, Doc should remember it (or write it down) until 1985, and therefore remember it in 1885, so there is no need for Marty to travel to 1885 from 1955.
In such case, complete plot becomes meaningless so producers in order to keep the script an production going on will ignore this nonsense and will try to give false explanations as they did in 4 possible explanations which all can be contested:
Ripple Effect - can not be applied in this case since there is no history alternation from moment in 1955 until 1985 (thus in 1885)
memory loss - would explain why Doc don't remember things, but is not a valid reason for Marty to start travel since Doc as scientist is expected to be sharp, but also to remember important and personal information (or write it down)
Doc ignores information - in contrast to Doc's character and previous decisions. As intelligent person, if Doc knows in some point of his future he should violently die, he would easily and minimally alter his decisions in order to avoid his fate, same as he did in BtF part I when he kept Marty's letter
Continuum self preservation - if this should be valid in the trilogy, then Marty, or anyone else, could never change the future by his acts in the past
Since scriptwriters are not providing explanation in the script itself, it is clearly a flaw in the plot, and the reasons it was not fixed could be it was discovered too late during production, or it was considered intended audience will not notice it.