(b) is another matter entirely. Maybe if their questing system wasn't tied to progression but it is. Apparently just letting people go back and replay old story quests would require resetting a players progression all the way back to that point. Its also the reason the Journal can't take into account story choices when it replays cutscenes. I complete overhaul of the questing system would likely be a pretty major amount of work.
In order to make any given storyline optional you just have to modify the prerequisite from the first HW (or stormblood) quest.

More to the point (b) would undermine one of the things I most love about FF14's story: the sense of continuation and progression of narrative. My character has a history and it is recognised and impactful on the world. I walk into 4.0 with all that history behind me and the allies I have made along the way and it sets the tone for the story I will play. Changing this is something I would most definitely be against.
The jumping potion already does just that, so I don't really understand what your point is.