Hello everyone! I tried to research this myself, but there were too many contradictions on reddit and the comprehensive rules be melting my brain. Hopefully you mighty wizards can clarify. :)

The relevant rules are 707.10 and 601.2i, of which succeeds rules 601.2a-h Most importantly, the relevant cards are [[Display of Power]] & [[Delayed Blast FireBall]].

I want to copy the spell once it is cast from exile so that the copy is doing 5 dmg to all legal targets.

If [[Delayed Blast Fireball]] is cast from exile and put on the stack, is the movement from exile to board not a payment defined under 601.2h?

My table was very hesitant due to the casting clause since copies aren't cast, but let it through. To my understanding and in simplest terms: I'm copying the spell on the stack as it is, not the literal card, right? So if it was cast from exile the copy would retain that characteristic and activate its alternative ability. Then the casting clause that is tripping everyone up would only be relevant for triggering things like [[Prosper, Tome-Bound]]'s Pact Boon ability, which wouldn't happen, yes?

Resolution:
I contacted a judge and this was my response:

R0b_ 15:58:48
The copy wasn't cast at all, much less cast from exile, so it only deals 2 damage