- The actual reason (Last spell has become the linchpin under these very specific circumstances)
Usually when enchantments stack, they are effectively separate, and the order doesn't really matter much in terms of disenchanting (from an ease of disenchanting standpoint, anyway).
Rhoda's logic only matches up with reality here because of the inadvertent linking created by Catalina being in Rhoda's pocket when Rhoda used the disguise spell.
And now, a confession: I myself am not 100% decided on what would happen if they tried to disenchant one of the other enchantments first. I have a few different answers to that question and logic for each of them, with "wouldn't work", "would work, but with greater difficulty", and "would work, but only if done a certain way", all being possible answers.
Fortunately, my view of canon is that "it's only REALLY canon if it's proven in the comic itself", and Rhoda's chosen wisely by accident, so I don't actually have to settle on what would happen if they tried to undo something else first (not yet, anyway).