Regardless of what Arial would like us to believe, we have multiple dev tweets that tells us the RAI doesn't allow for dispel magic to do what he claims. The RAW doesn't agree either. To agree with him, you have to ignore both RAI and RAW but favor his personal definition of a word which he's using to extrapolate multiple other arguments. But it all revolves around a single word.
You can't ready dispel to affect an instanteous spell. That's simply not possible. His argument requires simulation to be part of the game, time between attacks for example being a recurring theme. He uses the definition to justify that time must exist between attacks and because time exists you can do x, y, z. So again, you have to ignore RAW + RAI but favor his argument.
We also have proof via both RAI and RAW that attacks are sequential. But he claims that because the duration for these spells is instantaneous that it must mean they are simultaneous. Again, you have to ignore both RAI + RAW and favor his personal definition. He also uses this to justify other claims. If attacks aren't simultaneous, it means there's time between, so you can do x, y, z. So he tries to create a catch 22 to claim he's right no matter which way you go but the core of his claim is still faulty which makes this entire thing he's constructed faulty.
Each attack follows the Making an Attack rules and each attack follows the 3 steps. Multi attacks are only ever simultaneous if the game element in question specifically states that it is. That's how the system works.
There's really nothing more to say, especially if he's going to claim being right no matter what others post, + tries to shift the burden onto others and so on... To agree with Arial you have to abandon the RAW + RAI and favor his interpretation. I will never do that because this isn't his table where he can make houserules.