I've been mulling this over, not in disagreement. A thought I had is that another sort of "heart of 'story now'" is emergent dramatic premises, in respect of which the player protagonism drives or delivers a story that matters to us. (EDIT See my quote of Callois 1958 in my immediately following post for what I understand to be a direct implication.)
Many story now games will offer some system of bonds or commitments. Often connected to the reward apparatus. Passively, these can work just fine. Related to my first point, as GM I feel like you want to prep something that looks likely to stress these. You've covered that with the fundamental "learn what your player's concerns are for their PCs" and this takes it a little further to say to what ends.
The last decade or so has delivered (or made explicit) several powerful tools to RPG. Fiction first, as brilliantly articulated in AW and found in all PbtA. No-myth and player authorship, which go hand-in-hand and lay the ground work for players to really be authors (not just to establish the fiction, but to do so in the distinct way of forming a story.) Use of the metagame so that "when a player 'really means it' then their character's impact on play needs to increase" as perhaps best demonstrated in BitD. And goal-driven play (instead of adventure-paths or map-and-key).
Those in various constellations can fairly passively give rise to vanilla narrativism: in my experience they can allow successful drift and hybrids. I think story now wants to push it a bit further, so that some combination of situation, system elements, adversity management, and player advocacy finds out what happens when who you are and what you believe is cast into question. As you lay out, situation and system elements are thus important parts of facilitation.
Adversity management comes in here. I have some quibbles with prejudgement, even while agreeing with the not contradicting or overriding part. I don't think you rule it out, so I would just here rule in being opinionated (or grasping the opinion of the design.)
So my comment/question is really - dramatic premise - what about it? I could be wrong, but I feel like it's important to get in there explicitly. Note that I'm talking about a process or emergence. Some designs have certain types or natures of premise implied right in them, but that is without dictating what versions of that will emerge in play.