This is an interesting theory, but there's nothing in the rules saying that there is such an ordering of amounts of time. It could be that the game world allows you to make multiple targeting decisions faster than enemies can react to you Just Because.
It doesn't tell us what the ordering is, but whatever the ordering is, it applies to everyone. The readied dispel has an observable trigger: as soon as I see a beam emanate from the warlock, I'll dispel the
eldritch blast. Meanwhile, the warlock is saying that: as soon as I shoot the first beam, I'll wait to see what happens to the target and then use that information to decide who to target with my next beam. So, the dispeller needs only to see the beam to trigger his dispel, but the warlock not only needs to see the beam but also needs to see the results of the beam, then make a decision based on that observation. 'Seeing the beam' + 'the other stuff'
cannot take
less time than simply 'seeing the beam', no matter
how long you think each element takes! Especially as the dispeller only has to see one beam but the warlock must go through the entire process three times between four beams.
This means that
if the warlock has enough time to see the result of each beam before shooting the next, then the dispeller
must have enough time to dispel the still existing
eldritch blast.
Also, I don't think it matters whether or not you are still in the process of delivering your eldritch blasts; dispel magic says it can't dispel instantaneous effects, period.
Er, no it doesn't! It says that they can't be dispelled,
comma,
because the magic exists only for an
instant.
So it may be that, while you are making your targeting decisions, there's no actual spell-to-dispel, even though you're in the process of casting it.
You might have your jargon mixed up here. None of a spell's effect exist until the 'casting' has been successfully completed. Only when the casting is finished does the duration begin, and all of the effects take place within its duration. The
results of those effects are not limited to that duration, but the 'spell effect' is.
Maybe! The point of the text about them being undispellable is that, once the spell has affected a creature, the effect is not itself magical, it's just the way the world is. When you cast a spell with a non-instantaneous duration on a creature, there is magic affecting the creature imposing the continuing effect on it. You can't dispel the burns from a fireball or the damage from an eldritch blast.
Under 'Casting A Spell' on p202 of the PHB, it says:-
Each spell description in chapter 11 begins with a block of information, including the spell's name, level, school of magic, casting time, range, components and duration. The rest of a spell entry describes the spell's effect.
So, what is the 'spell effect' of
eldritch blast? Is it, "Choose a target and it loses 1d10 hit points"? No. The 'spell effect' of
eldritch blast is:-
A beam of crackling energy streaks toward a creature within range.
That's the 'spell effect'! The 'spell effect' is
not "A beam of force instantaneously in your hand and then the spell ends. You can throw it at a target any time you like".
The 'spell effect' is that a beam or beams instantaneously streak from you to a target or targets. The streaking beams
are the 'spell effect', and like all 'spell effects', like it says on p203 under 'duration', "A spell's duration is the length of time the spell persists".
If, for some reason, you envision these instantaneous beams as hanging around, waiting to be launched one by one with enough time between them to see what happens to one before choosing a target for the next, and this bunch of beams (which are the 'spell effect') exist entirely within the duration (which they must), then this means the duration doesn't expire until the final beam has been shot. This means that, for this 'instantaneous' spell, the magic
does last long enough to be targeted by a readied dispel!
Of course, I believe that having beams hanging around is inconsistent not only with 'instantaneous' (because, among other things, if it worked that way then it could be dispelled, and since instantaneous spells cannot be dispelled
because the magic exists only for an instant then the spell cannot work that way) but also with the description of the 'spell effect' which simply says that crackling energy streaks toward your targets, not that beams appear and hang around near you, waiting to be directed later. If it said that, then the duration would be 'one round' or however long the beams hang around for.
Sure it is. This is a game. The rules can be anything that the writers think is playable and fun.
You have touched on a point that I recently mentioned. The only way that JC could say that instantaneous spells could be spread out in a characters turn while readied opponents watch helplessly, unable to follow the rules about readied actions, would be if he accepts that he's left any realism behind and just wanted to rule in favour of casters without any attempt to simulate the 'reality'. We know he did this when he wrote the rule about choosing to knock your opponent unconscious
after you discover that your deadly attack killed him. However, his knockout rule really does make the game more playable than other systems such as 'striking to subdue' or 'non-lethal damage'. But there is nothing more or less playable about choosing targets simultaneously or consecutively. It would be an unneeded divorce from reality for no real gain in playability.
