Is my Xenon Damage Calculation wrong?

Doesn't "total damage" affect the overall damage output, not the damage range?

Also, an off topic question but does Multilateral stack for all tiers or does it use the highest one?

Total damage skills have pretty much always affected range, prior to Unleashed, only total damage bonuses from equips were not reflected in range, but they are now. And yes, all the Multilaterals stack with each other.