Originally Posted by Jennifer
Plus Marty is the last of his siblings to disappear in the photograph in the first movie, which could be explained either by simply the fact that he was the last to be born so the ripple would reach him last, or that since he's traveled through time the timeline change is catching up to him slower than it did the rest of his siblings.
I think your first guess makes the most sense. Dave is older than Linda which is probably why Dave starts fading before Linda does. But your second guess holds water as well as marty fades in a different manner; Dave and Linda have different body parts fade out in sequence whereas marty's entire body fades.
I'm not sure what the thought process for putting Mr. Fusion at the end of part 1 was (remember no sequels planned at this point). Probably so that doc wouldnt have to put on a radiation suit to put the plutonium in just for what they thought would be a gag. Or possibly due to the humour of seeing him go through the trash. But part 2 and 3 were conceived at the same time. So I'm sure they had already considered a problem to have them stuck in 1885. Actually one of the first drafts of the second film was to have Mr. Fusion damaged in the hoverboard chase and have Marty and doc stuck in 2015 similar to part I with the plot to drive the delorean into the grand canyon to get back to the present.
But anyhow it's clearly shown in the first film that the plutonium needs to be put in for each time travel. Mr. Fusion makes this easier and may possibly allow for multiple time travel without needing to be filled. I'm sure zemeckis and gale realized that if plutonium was not accessible in 1955 the first time around, it certainly would not be a second time around and definitely not in 1885.