Yahenni's Expertise

Effects that allow you to "cast" a card don't allow you to play a land card.

If a creature's toughness is reduced to 0 or less by Yahenni's Expertise, that creature is still on the battlefield as you cast your free spell. It can be the target of that spell, although it will become an illegal target before that spell resolves. If it has any abilities that trigger on the spell being cast, those abilities will trigger.

Any triggered abilities that trigger while performing the Expertise spell's first effect won't be put onto the stack until after you're done casting your free spell. They're put onto the stack at the same time as any abilities that triggered while casting that spell regardless of the order in which those abilities triggered.

If you cast a card "without paying its mana cost," you can't choose to cast it for any alternative costs, such as emerge costs. You can, however, pay additional costs. If the card has any mandatory additional costs, such as that of Cathartic Reunion, you must pay those to cast the card.