That being said, the Basic Rules lay it out pretty well in the Adventuring section:
(Paraphrased)
1. DM describes situation.
2. Players say what they want to do.
3. DM describes resolution.
Now, the DM can treat everything like a battle scene, and it drives me nuts when it happens. Because: miniatures aren't necessary for walking around non-combat scenes, I don't want to make a "check" for everything, and hit points are awkward. I have definitely held an opponent at arrow/sword point in the past, just to get drawn into a long combat scene because: hit points.
To me reciting the basic player-GM dynamic as far as resolution doesn't do a thing here.
The GM can decide that one swing of my character's non-magical axe produces butterflies, another heals the target and an third damage the target and another throws fireballs and all that fits within that same set of three stages you just described.
The Gm can decide my character running across the street caused a world to die one time, causes women in a stream nearby to get preganant in another, gets me across the street quickly another time and even wipes out all elves in the multiverse in another... all still within that same three stages.
So, the fact that the rules "lay it out pretty well" says nothing about whether or not any of this was their intent or whether or not any of this is something expected.
Similarly, the decision to bypass essentially the character build and specifically the character defenses in order to get the resoution the Gm wants to happen is not "helped" by citing that broad description.
To me, unless we have had a strong clear up-front description of such additional "skip mechanics" cases (likely with equally clear ways we can use them as players) and acceptance of them, this big of a breach of the expected playstyle and mechanics of how things work in the game is a huge breach of trust between Gm and players IMO in most if not all cases.
if this were done to force a "obey or die" takeover of player control, i would choose "die" and then vote with feet.
trying to promote or prop this kind of thing up by citing the basic sequence of play and the general GM does anything they want falls way short of the mark for me.
The players are participants in the game, not playthings for a Gm to slap his giant GM three-step in the PHB cahones across our faces whenever they think it would be real cool to do so.
When we started playing in DND 5e, we took on a broad set of expectations and mechanics - an agreement between players and GM. Ther sure might be a lot of house rules and rulings and, yes, there is a broadly stated Gm option for stepping outside the rules but for a Gm to do so to setup a "die or give up" situation with new "NPC spring on you mechanics" is way outside of that agreement in spirit if not in fact.
In the case where there was not "some semblance of this approach of out-of-combat-bypass-all-damage-direct-to-dead was not agreed to explicitly before the game" i would urge all the other players to choose death as well resulting in all characters dead issue for the GM to deal with. I would also likely have another campaign ready and inviting players up within a week.