I am not sure what you are meaning by "hold up" with fireball.How exactly do you hold an instantaneous spell up while all that happens?
Edit: Or to put it another way, if you can hold up the instant rays while you surge and attack, you can hold up a fireball explosion while you surge and attack, finishing the explosion after the attacks. The duration of them is the same, so those two spell actions would be equally divisible.
And that's perfectly valid. The rule of cool works well for people who like to play that way. My point is that actions are not naturally divisible. They need a specific exception to allow them to be divided, and there are plenty of such exceptions in the rules.
Action Surge does not "hold up" anything. It gives you an extra action when you take it.
Fireball and Scorching Ray are both instantaneous durations but the former has one event - one resolution - it goes off. The other has sequential resolution - several different things happen in order and its permitted to make the choices, resolve one, see what happens then make the choices and resolve the next.
The rules already allow for say reactions to change things between shots of the scorching ray that could not cause the same issues for fireball. A Hellish Rebuke might drop me after one scorching ray shot, before the second shot was even declared - end done. The same rebuke from bring fireballed does not stop the res of the fireball victims from being hurt by it.
Instantaneous duration foes not mean simultaneous resolution or evdnts/effects - which is why magic missile and its explicit simultaneous internal to the spell gets singled out.
But if you want to give more clarity in what "hold up" for fireball means, go ahead.