Yes, Leebo's ability would absolutely trigger, I agree with you there. But wouldn't the clause "When you are suffer critical damage during an attack, you are instead dealt the chosen Faceup damage card." override Leebo's choosing between 2 cards, the same way is overrides Maarek's choice between which 3 to deal? Regardless of the one he picks, the condition card forces the assigned one to be resolved instead.
The order of events would look something like:
1. Leebo gets ISYTDS.
2. Leebo suffers a crit during an attack.
3. ISYTDS deals the damage card, possibly bypassing shields.
4. Leebo draws another damage card and picks one of them.
5. ISYTDS overrides Leebo's choice and forces him to suffer the assigned damage card. The other card is discarded.

