It might help to look at when decisions are being called for. Under my approach (T is for time, not turn) -
T1 = DM: based on how things have unfolded, let's roll initiative
T2 = Fighter Player: rolls lower than DM
T3 = DM: decides what orcs do, and resolves their turns
T4 = Fighter Player: decides what they do, based on the current game state (groans weakly, dying)
T5 = Healer Player: decides what they do, based on seeing the current game state (fighter groaning weakly, dying): casts
It sounds like you might be describing -
T1 = Fighter Player: decides what they want to do
T1' = Healer Player: decides what they want to do
T1" = DM: decides what orcs will do
T2 = DM: based on how things have unfolded, let's roll initiative
T3 = DM: because orcs won initiative, game state is unchanged so unproblematically resolves what they decided to do
T4 = Fighter Player: what they decided to do is now at odds with the current game state: insert sound of SoD shattering
T5 = Healer Player: didn't decide to cast healing because no one needed healing back when they decided: sound of SoD shattering intensifies
Can you correct the above, which I feel sure I must have laid out wrongly?!
I don't understand your schema. Are the times
times at the table or are they
imagined times in the fiction? And what is the meaning of the "prime" markers on T1? And are you asking about my description of D&D turn-by-turn combat, or my description of a system that would not have the same issue?
In any event, I've already posted an illustration of the issue upthread; so has
@FrogReaver. I'll set out another.
We have two characters in the initiative order, P (controlled by a player) and N (controlled by the GM). They are separated by 25', a distance which either an cover with a single movement. N is at the top of the initiative order. Here are two possible sequences of events that follow.
(1) The GM decides that N holds their position, and shoots P. The attack hits. P's hit point total is adjusted. The player decides that P takes some sort of healing action in response to being shot.
(2) The GM decides that N holds their position, and shoots P. The attack hits. P's hit point total is adjusted. The player decides that P closes to melee with N.
According to the rules of the game, both (1) and (2) occur over a 6-second period. The rules of the game (including the rules for movement, opportunity attacks and similar) also establish that it is possible, in some circumstances, for P to close the distance to N and get too close to N for N to shoot effectively. (This would be the case in a scenario that resembled (2) but the initiative order was reversed.) That knowledge informs our understanding of the actual scenarios.
So in (2), something must have happened that made it possible for N to get a shot off
before P got all up in N's face. You have suggested one possibility:
P stumbles. N's choice In the fiction) to shoot rather than, say, fall back, is informed by observing that P stumbles.
But at the table, we cannot establish the fiction that
P stumbles until after N's turn is fully resolved, and we now come to P's turn, and we have to posit some fiction that will explain why P couldn't get to N before N got a chance at a clean shot.
That is reinforced by (1), where there is no narration that P stumbled, because P doesn't move at all. In (1) P's action clearly follows, in the fiction, on N's - N shoots, P treats the resulting wound - which does make one wonder
what is N doing while P treats their wound? Perhaps at the top of the next initiative order we will come up with another bit of narration that explains that - maybe N is taking a long time to nock another arrow? - but we won't be able to narrate that either until after the GM declares N's action. Yet in the fiction whatever that is is supposed to be a (partial) cause of whatever action it is that N takes!
I hope that I have made the point reasonably clear.