Well, look, the original story outline obviously had Doc dying of tuberculosis, and when those seasons disappeared, to be replaced by a two hour movie depicting a 13 year interval from the beginning of the story, the writers had to make a choice; do we bring back a terrific, and known, character like Doc, or do we spend some of the 120 minutes developing new characters? I think the reasonable choice was made to only have one new character, and do some hand waving about a misdiagnosis of Doc's condition, or miraculous recovery.
With regard to Jane, as much as I loved the actor, character, and show, I always felt they didn't quite nail how her alcoholism affected her; just a little too over the top. If I had a conversation with a healthy David Milch, I'd ask him about his backstory about Jane. In the 1st episode, Wild Bill references her ability to mete out violence to those that displease her, but then we later get scenes where she is frozen in terror by the mere sight of Al or Cy. Now, obviously this is psychologically explainable, especially today, with our insights about how past trauma can produce contradictory behavior. This is such a rich, hugely complex world that Milch created, that it's hard to fully explore everything. I appreciated Jane showing what Hickock referred to in the first five minutes of the show, 15 years ago.
Milch, and whomever assisted him, faced a tremendously difficult writing task here; how do we pull this very complex world back together, showing an interval of 10-plus years, and get a resolution in 120 minutes? Would I have liked it if they had 3 times as many minutes? Yes, but I think the end result was really good, given the task.