I've been playing Illaoi a bit lately, but I'm still not entirely sure how her Q works.
If it hits a champ, it restores 5% missing hp. But if one tentacle hits 2 champs, is that 10% missing hp? And if two tentacles hit one champ, is that also 10%? Then, getting more complex, one champ getting hit by 2 tentacles, then another champ being hit by one of the tentacles is 15% hp?
Thank you for your answer is you did :) 
It says, and I quote:
Illaoi is healed for 5% of her missing health whenever a Tentacle damages a champion.
It does not say anything else, and nothing like anything you wrote. So: "No."
diminishing returns.
you have 1000 missing HP.
You hit 1 champ, and gain 5% of your missing HP - 50 HP
you now have 95 missing HP. Let's hit another champ. You now heal for 47.5 HP.
As I understand it, (from playing Yugioh for many many years), the effect triggers when you hit a champ, but doesn't check how many champs you hit. As such, hitting 2 champs will still only net you 5%.
Let's try the last scenario: remember, each hit with a tentacle is 5%. We'll stick with 1000 missing HP. first hit against first champ: 50 HP healed. second hit against first champ: 47.5 HP (97.5 total) hitting another champ: 45.125 HP (142.625 total)
this works out to 14.2625% over 3 hits.
Again, since it's 5% of missing HP, not max HP, diminishing returns.
A single tentacle will only heal Illaoi for 5% of her missing HP, regardless of how many champions are hit. So a single tentacle hitting 2 champions will still only heal for 5%.
On the other hand, if multiple tentacles hit a champion, then each one will heal you for 5% of your missing HP. So if you get 4 tentacles to hit at least 1 champion, you will heal for 20% of your missing HP.
