About the Elixir restrictions thing:
Elixirs still replaced each other if you activated them. You could still have one of each in your inventory, but you couldn't have more than one of any individual Elixir in your inventory. It also didn't prevent you from rebuying an elixir that was already active
I suspect it was a leftover from the tested Endless Elixirs rune ("elixirs can be stacked and do not expire") requiring you to be limited to one of each Elixir type. Item exclusion rules like this are handled by assigning items to a "group", and one of the many features of a group is the ability to limit you to a set number of items from that group at once (usually just one).
However, groups can also have their limits overridden by scripts (this is how Cassio passive locks her out of boots, by setting her Boots limit to zero, as well as when Arena had an augment allowing any number of Mythic items, by setting the Mythic item group limit to six). I imagine Rio...