Huh?This is why there's a problem. If there wasn't a problem, you wouldn't have to invent an entirely new "phantom rule" that says you can divide one Cast a Spell Action(with multiple attacks), but not another Cast a Spell Action(resolves in one resolution). If you can divide the action, you can divide the action regardless of whether it's one resolution or multiple resolution. If you can split up Scorching Ray to use a completely different action with Action Surge, you can do it with Fireball as well. So if actions are naturally divisible like you say, you can take the Cast a Spell Action, and the split it up by casting fireball, then while it is traveling towards the enemy, stop it in mid air so that you can take your other action, then have the fireball resume its travel and detonate.
The game rules provide already for the differences between how scorching ray plays vs fireball. One is a sequential series of attacks and damage the other is a single event. I dont have to invent a mystery rule there, it's already rule. I can point you to actual rules in the book that describe in detail how sequential attacks resolve. I can point you iirc to sage compendium follow-ups and clarifications as well. No "phantoms" here.
Contrast that to the phantom indivisible rule which has apparently got a lot of only here and not there filters in it's incredibly detailed non-write-up.
As for holding up fireball mid-air, before it detonates, cant really address that unless you give more info on the interruption. There are certainly more than a few interruption type events by rule that could occur.*
But by RAW fireball is one event/resolution and scorching ray is multiple sequential so that difference is not a phantom rule, it's just the rule by the books when they describe each.
*Most likely if you wanted a rule of thumb in a vacuum, I would say that its gonna be similar between fireball and scorching ray for "hold it in air" to fireball being similar to one ray shot of the scorching ray - in terms of kinds of interruptions. Obviously one is an "attack" the other an AoE so different features may trigger.