k so first, there is no look phase to the spell. You determine the target of an attack when you make the attack. That's simply how game works. What you're asking/talking about is not game mechanics:
1. The length of time between attacks is not defined anywhere in 5e.
2. The same character can make 11 attacks, move, use a bonus action or 1 attack, move and use a bonus action within the exact same stated time a turn takes: 6 seconds.
3. Time, as you're using it to justify attacks happening simultaneously, is not defined within 5e.
4. It is not stated anywhere that time should be a factor when resolving a single or multiple attacks.
5. The instantaneous duration description found in the PHB does not list a restriction in regards to time, does not define anything beyond the fact that instantaneous spells cannot be dispelled and doesn't support your claim that attacks are simultaneous.
6. The EB spell itself specifically states that all attacks are to be resolved separately.
If anything it's a simulationist claim that is trying to change the rules of the game (because 5e doesn't care about this subject at all). You require that there be time in between attacks of EB, so you look for reasons why time should be there and tie in all these other things (some invented and some not) in order to make your claim real. But it doesn't work that way. What you should be doing is first looking to see if your basic claim (that there's time in between attacks and 5e cares about this) is true. It's not, as demonstrated above, outside of your houserules and home game.
As stated before, actions in 5e CANNOT interrupt each other outside of specific examples like the spell Shield. You cannot use a readied action to interrupt another action. You cannot interrupt the Attack Action of a level 5 fighter in between his first and second attack with a readied action unless that readied action specifically states (like shield) that you can. Readied actions as the kind a player can ready by declaring a trigger and action they are going to take in response to that trigger cannot get around the general rules which govern how actions interact.
This is an example in steps:
1. Declare I use Eldritch Blast
2. Use up the action type associated with Casting a Spell and Eldritch Blast (my action)
3. Use any material costs associated to casting the spell I have chosen and perform any other requirements for casting the spell.
4. Finish casting the spell (this is the last step where the spell can be counterspelled because after this you are resolving the effects of having successfully cast the spell)
5. Resolve the effects of the spell (in this case making x number of attacks as described in the spell effect section, say 2 attacks for this example)
Making an Attack
1. Declare target of attack 1
2. Figure out the modifiers if any for the attack
3. Resolve the attack (including damage and effects like forced movement for example)
1 Declare target of attack 2
2. Figure out the modifiers if any for the attack
3. Resolve the attack (including damage and effects like forced movement for example)
Any readied action by an ally would begin now. However, as stated a Shield spell could be used during step 3 of the attacks (Resolve the attack) when you are declared to be hit. This could lead to the attack instead missing, but only because the spell itself allows for this exception to the general rules.
As for your last example, that is correct. You cannot interrupt the action of another creature with a readied action. Immediate Interrupts from 4e no longer exist. Reactions in 5e, outside of very specific examples, work like Immediate Reactions which always happen AFTER their trigger is fully resolved. In this case, the action being used to attack. Note that the creature attacking you moving away would normally provoke an Opportunity Attack which you can perform instead of your readied action. If you have a feat to stop the OA, that's a feat benefit and imo it should be self explanatory that feats are powerful and should remain powerful. But the fact remains that you had to give the attacker a feat to circumvent something you obviously knew would happen to mitigate the strategy you suggested be used to counter the argument I presented. So

to you.