Specifically, Dark Kael's A1 reads "Each hit has a 25% chance to instantly activate...". And, even more specifically, I'm wondering whether Sniper, which reads "Increases the chances of placing any debuff from Skills or Artifacts by 5%", will work on that A1.

Ordinarily I would have said no - but, that same skill also has a booking bonus of "Buff/Debuff chance +10%".