Well, you introduced a new set of 'maybe it really works like this and that would mean...' and Arial responded with 'no, maybe it works like this and that would mean....' Both were entirely conjecture. I just made my comment that you don't have the rhetorical high ground to dismiss his conjecture while pushing yours -- both are unfounded in the rules.
As for what Arial posted (recently), that seemed to follow the rules pretty well and ask 'what happens here.' Granted, he provided his own answer, which I'm not sold on, but I think he did a good job of presenting the breakdown and question up to the point he provided his own answer.
I've bolded the part that's factually wrong, and thinking about it you're overstepping a bit by saying that I was only posing a hypothetical.
I will restate the order of events to clarify.
1. Ariel Black is complaining about not being able to dispel Eldritch blast because he's damn sure that seeing what your first beam does and then using that in your decision for the next target is not instantaneous.
2. I respond with an expanded form of "you haven't covered all of the possibilities here," except more entertaining because the example I came up with resonated with me.
3. He immediately retorts by citing the PHB and then continuing as if he had quoted different text.
4. I call him on that.
5. You say this is all hypotheticals.
I suspect that hideous quote chains back to the source would have helped you here, but this forum thankfully doesn't default to that sort of layout travesty.
I have, at all points in this series of events, been shooting down claims that he made. His options were not exhaustive, and his quotes did not say what he claimed they said. Supporting my hypothetical structure for a high level eldritch blast only matters in as much as I am supporting the first refutation.
In that case, what on Earth are you talking about?
It's a cause-effect relationship. Casting the spell (the VSM components) causes the spell to come into effect. The cause comes before the effect; it literally cannot be another way.
Until the casting process has been completed successfully, it has not caused the effect yet, because the required components have not been executed yet.
I know what I mean, what do you mean? What do you think the sequence of events is? Where do you disagree, exactly?
You were acting like there were only these few options and I've told you no, there's another that's compatible.
Maybe what I'm suggesting will be clearer if I change how I'm referring to the spell.
We have level 1 EB, Level 2 EB, Level 3 EB and Level 4 EB.
Casting level 2 EB is exactly the same as casting level 1 EB, but maybe you move a little more skillfully (faster, more efficient) so that you can tack on some flourishes at the end. Speak another word into your FusRoDah. Level 3 EB is just like level 2 EB but with more verbal and somatic stuff happening after you're already through the first two stages. I will leave iteration up to level 4 EB to the reader.
This is one spell. You weave the weave and all that one time, it just so happens that the lower level variations of the spell fit into the higher level one, sort of like how a 4 arrow quiver is just like a 3 arrow quiver, except that after those 3 arrows there's a 4th one, or like how a 4d10 firebolt is just like a 3d10 firebolt, but with a 4th die after the third.
If by some means that we've all yet to agree on over the course of this thread, you're made to stop waving your hands or talking or casting a spell at all, then you went through all the motions to cast a lesser level of EB, and since you didn't shoot 4 lasers it would seem that you didn't cast that version so much as one of the lesser ones.
Now, I don't have any special need to convince anyone that it definitely works like this. This is just an option that really really doesn't mesh with your idea that your cast spell action breaks down into a discrete components stage and then a magical effects stage after that.
As for what I think- I don't think that there is a sequence of events. From the table perspective the DM rules on how slight of a thing you can react to ("I wanna ready an action to cut off his pinky finger the second to last time he undulates it before the spell goes off." "You're not a mage, you don't know anything like that much detail about the somatic components of the spell he's casting, and just do it now instead of readying an action you pain in the ass.") and if that's on any scale smaller than the book has explained then the DM works out how that works. This has only ever been an issue at a table if somebody here has actively tried to make it into one, so of course the rules don't waste time re-explaining that instantaneous spells are instantaneous.
Aaaand it's fallacious to ask for me to solve this. I'm only making the claim that your explanation was lacking, there's no burden of proof here to force me to propose a working solution.