I suppose if there are 2 Delorians in 1885 the problem with Doc cannibalizing one is that he has to leave one in good working condition for Marty to discover in 1955 in the first place.
But I would think they could get around that by paying Tanner $80 (not easy in 1885 but doable), having Marty leave, then Doc can fix the car in 1885 and leave. If Marty was feeling adventurous he could even go to 1985, get the parts Doc needs and go back to 1885 and give it to him.
Then they leave behind a Delroian in good order for Marty in 1955 and just leave together to get back to 1985.
Seems easiest.