Sorcery Interaction - Weapon vs. Casted Spell?

Does Iskandra’s Elemental Exchange effect other sorceries you cast, such as Panetti’s Replicating Missile?

No, at least not normally. A skill needs to have %weapon damage for this kind of buff to apply. There are items that give some amount of %weapon damage to Panetti’s Replicating Missile, but ~40 flat damage from IEE through 18% weapon damage is what, around 7 more base damage? That’s comically small. +1 to Panetti will give you more.

To elaborate on Stupid_Dragon’s answer a bit more - the x Elemental damage and Energy Leech does not apply to skills without % weapon damage like Panetti’s Replicating Missile. The +x% Elemental damage applies to any instance of Elemental damage.