I think Zoe should be unable to pick up summs that she has locked in
Let me tell you precisely why you DON'T want it to work this way:
Right now, Zoe has 11 potential drops from balloon minions. Let's imagine it worked the way you want it to. I can now do three things.
First, I can reduce this number to 9, artificially inflating the likelihood of any particular drop from 1/11 to 1/9.
Second, I can manipulate my drops into ones that are statistically more useful than ones that are not.
Third, I can make the game safer for me through my choice of Summoner Spells and simultaneously make my drops more aggressive, or vise versa.
What do I mean by this?
For example, I can take Cleanse and Exhaust, two Summoner Spells with shorter cooldowns than Flash and ones that aren't very exciting to get from minions. By doing that, I completely remove them from her drop-table. Now, I've made the game safer for myself by carrying defensive Summoner Spells, and I've made it more likely that I get things like Flash, Ignite, Redemption, Smite, and Proto-Belt from balloon minions. Or if I go the Flash-Ingite route, it's now more likely that I get defensive drops like Ghost, Heal, and Barrier.
Is this what you want? A system that is exploitable to Zoe and let's her tilt the random drops in her favor?
Right now the odds of any particular drop are 1/11. It's simple, it's fair, and it's consistent. Don't mess with that.