Priests don't have castable spells that shoot projectiles, unless you talkin about penance. Only lock's and mages have projectiles, some shamans and hunters. The goblin here is wearing a priest spesific set, but is not a priest. I suggest more background information to be done.
Yeah, that's right, Priests don't really have any offensive projectiles. Whether the spell in the piece is a projectile or just the "casting animation" could be up for interpretation, but I can also be wrong.
Is there a lookalike transmog for the one that the goblin is wearing??
And lastly, do projectile-spells and instant-spell (not instant cast) have any differences other than the time it takes to reach the target?
Yes. If a projectile spell doesn't reach you, it cannot damage you. Thats how it works If you drop a clone while a Chaos Bolt is flying towards you, you avoid the damage. But instant casts OR insta-reach spells (Like the 1.5 sec cast timed Arcane Bolt from Mages) will affect their target instantly.