Re: Alternate timelines - What can you mess with???
I only skimmed above but I would say nothing is really taboo or off limits. Everything all depends on presentation and your skill as a writer. Another thing to consider is what causes of the deviation in the timeline from actual recorded history, please note as a history major i didn't use "true" history, and your arch.
For example Kim Harrison's Hollows books are set in a world were every thing deviated in the 60s due to a genetically altered virus accidentally making its way into genetically altered tomatoes. These tomatoes then make it across the world due to mass transit and and wipe out the larger portion of the human population and only the intervention of the "Inderlanders" or supernatural beings coming out of hiding help save everyone. This sets the stage for the current timeline and explains why Elves, Witches, Weres, Demons and the like exist in todays world and why things are different.
Basically why does the timeline split?