The triggers can't accurately detect when your attack hits. Thus it checks when lanaya starts an attack, and then detects when lanaya deals damage to her target. This is assumed to be the attack, but if you start an attack from meld and dagon the target before the attack hits, the effects50*level damage, -armor, red text
of Meld will be applied earlier.
Since meld is based on windwalk, the attack that breaks out of it will be a buffplacer that overrides any critical strike.
Conclusion: You can't crit on the attack breaking meld.