Question about Fireball Mechanics Explenation.

Hi! Anyone can explain me, or give me a link to complex elxplanation how Fireball work?? I see fireball can work it three ways and i have some question about this, i try to separate all to make it easier:

Before we start: I try to build a fireball setup that will focus on Explosion damage and for fast clearing map's im gonna use Herald of Ash.

1. When on PoB we see separated Projectile damage and explosion damage it was stacable? Lets say:

Proj. DMG is 1000 and Explosion 1500, so in the end when shaper get hit from them it will stuck 2500 dmg? And of course after all ignite that adds extra 4 second of burning every second let's say 500 dp/s so at the end:

1000+1500+(500x4)= 4500 dmg?

Is that correct?

2. If i manage to hit a shaper with 2 of 3 fireballs, should i get outcome dmg and get only 1/3 or 2/3 of them, or outcome damage is for one of the 3 fireball's.

3.What will scale a fire ball better, Proc damage or spell damage (i know i can figure it out on my own but maybe here is somone who test it out already.

PS. I know there is already fireball build based on ignite dps, i'm not iterested on that one, also this golemancer one. I'm gonna get explosion one so pls focus on that :)

HAVE A NICE DAY! And may exalt's be with You Exiles!