Events within a turn are sequential, yes, but all the turns within a round overlap and happen at roughly the same time. Or do you imagine that the participants in a combat are waiting around not moving or taking actions while each character takes its turn in order?
"Events within a turn are sequential, yes, but all the turns within a round overlap and happen at roughly the same time. "
I submit to you the vast vast majority of the mechanics in the game and how they actually play out in play to counter your claim that all the turns play out roughly the same time.
My dagger example is an example of 3-4 turns within a round that illustrate the failure of that perspective.
"Or do you imagine that the participants in a combat are waiting around not moving or taking actions while each character takes its turn in order?"
Yes, YES YESYESYES... since that's the only other possibility it must be true. Just like the fact that we rarely show on screen player characters taking a dump means Rangers don't poop.
Wait, maybe there is another posdibility.
What if, like ranger poop, things and activities exist in the game world that are not shown on screen?
What if the activities within a character's turn" are just say a highlight we show - like the most significant?
Now lets run with this radical heresy. Let's see where it leads.
That would mean folks are not standing around waiting their turn but are tied up with other stuff, even maybe deciding, maybe not getting hit etc. Yeah that plays out like a lot of action scenes we see and read. Each character gets a spotlight moment back and forth but you still see stuff in the background.
Also, look at AO, where an action on someone else's turn lowers their guard and you can land an extra blow. That seems to fit the model of "off screen activities" where the two are swinging, thrust parry etc in between their "turns" instead of your incitful "Or do you imagine that the participants in a combat are waiting around not moving or taking actions while each character takes its turn in order?" Idea.
So, in spite of how appealing your two possibilities AAndB are I think I will go with option O for Old we have been doing this for long long time...
Option O
The turns in a round are sequential, not simultaneous, so the mechanics and the narrative line up.
Off your turn, you like everybody else is tied up with non-highlight non-results-in-stuff-worth-screen-time activities. A lot of punches that dont matter for example.
Some exceptions like AO and other reactions spotlight some off-turn activities that produce results worthy of screen time.
Now let's test this theory.
Long corridor ending in T intersection.
Halfling will be crossing the t. Dwarf 90' down corridor.
Others involved in fights etc.
Round 1
Halfling walk out of left side of t, looks down corridor, firebolts the dwarf then finishes walking to right side of t out of sight.
Dwarf then wants to shoot crossbow at halfling but has no shot due to - you know - wall.
Does that fit simultaneous model? Well, no.
Does that fit sequential modrl where its assumed when the halfling walked past the dwarf was involved with or focused on other stuff? Where the dwarf did not have time to shoot until after the halfling moved out of sight?
Now, of course, the dwarf could now decide to ready action to shoot the halfling as he expects them to repeat the maneuver next time. It that shows spending an action to focus on the halfling while dealing with other stuff.
So, got a say, the flurry of activity and sequential screen times seems to me to be a much more appealing mesh of narrative and mechanics than either the "call it simultaneous but in name only" or your awesomely clever and tempting "the participants in a combat are waiting around not moving or taking actions while each character takes its turn in order?" Alternative.
So, of course each gm chooses their own.