Here's the problem, though. You also get to choose when you take your actions during your turn. By arguing that actions are naturally divisible, you are arguing that a fighter can run up, swing his sword for his first attack, move 5 feet, action surge, take an entirely different action, continue moving 10 more feet and finish his second attack. You are also arguing that a fighter/wizard can move 10 feet, cast Scorching Ray that has a duration of instantaneous, divide that action up and action surge after the first ray, swing his sword at an enemy, move 20 feet(despite being unable to move in-between spell attacks by RAW, because now we are in an entirely different action and you can move in-between weapon attacks), finish up his scorching ray strikes, and then swing a second time.
There's a reason why you have to have an exception to the "phantom rule" in order to do something else during an action.
"Problem"?
I see "examples" but not "problems".
You describe move-attack-move-surge-attack as if its done obviously flawed thing. You just left out what the "problem" was.
You mention the scorching ray and not bring able to move between spell attacks by RAW and dont say how that is a problem.
"(despite being unable to move in-between spell attacks by RAW, because now we are in an entirely different action and you can move in-between weapon attacks), "
Now, let's deal with that one, ok?
You can move between spell attacks in RAW.
You can.
Really.
There is no rule stopping it.
Not one.
Now, what you cannot fo, by RAW, is use the Movement Between Attacks rule to split your normal movement in an action between attacks of a spell because ***that rule** requires weapon attacks specifically.
In your example *that rule* is not being used to move between spell attacks, right? The rule being used is the action surge giving you a new action and whatever that gives you.
See, that's the key, there is no general rule which forbids movement by any means between spell attacks. So, no rule is broken by action surging between spell attacks.
Let's give you an example.
We already know with scorching ray we can pick a target, fire, see results, pick target, fire, see results etc. Nothing RAW forces those "instantaneous" spells with multiple attacks to not be done sequentially (as confirmed by Sage whenever asked.)
So I move, fire, but if that shot triggers say a reaction like dissonant whispers I may be forced to spend my reaction and move away right then and there - but I still have the ability to choose my target and shoot from that new spot.
That did not violate the MBA rule because ts was these other game features that caused the movement, not the MBA.
Would you force that Scorching Ray player to forfeit his shots because DW made him react move away? Or would you rule that a reaction triggered by "hit by an attack" would have to wait not thru just one attack in a sequential set but all of them?
The reaction movement away did not violate RAW because I did not have to use "moving between attacks" rule to move. Just like if I action surge between scorching ray shots no rule was violated.
But the long and short of it is, the rule for MBA does not establish any global prohibition against movement between spell attacks.
Matter of fact, IIRC JEC answer to the question of moving between spell attacks was that there is no general rule allowing it... which obviously leaves open the cases where a specific rule can allow it.
Movement Between Attacks
"If you take an action that includes more than one weapon attack, you can break up your movement even further by moving between those attacks. "
So, again, we reach the key point to me in all this.
You listed two EXAMPLES that you described as problems that would occur if we did not imagine a phantom discrete indivisible action rule, but hey, guess what, we can actually look at honest to goodness real RAW written down rules to go thru and see "were any rules broken."
At no point is any phantom rule needed to resolve this.
That's because the rules being used - using bonus actions when you choose, MBA, sequential attacks - are actually printed rules we can read (and in some cases - printed clarifications)
But this amazingly finessed phantom indivisible discrete action rule whose text is somehow known to be so precise as to divide between "when you choose" and "any time" and between some bonus actions but not non-actions requires us to based rulings on that phantom rule we cannot see, cannot read and cannot examine the wording of.
YMMV.
But in my game, when you action surge, you get a new action, right then and there on your turn. No phantom indivisible need apply. In my game, you cannot use MBA to move between Scorching Ray shots cuz they are not weapon attacks, but there is no problem with movement between them as a result of other features, events and effects and if there are questions we check actual rules for guidance, not phantoms.
