Regarding the zeta on Talzin's unique:
My understanding is that if a single 'hit' defeats a character that was previously above 50% health, then any effects that occur when they drop below 50% health will not trigger. However if that same character is defeated via multiple hits, and any of those hits drop that characters health below 50% at any point during the attack, then those effects will trigger.
For example, I believe that Sabine would trigger those effects if any one of the hits from her basic dropped her target below 50% before ultimately defeating it.
Regarding the zeta on Talzin's leader ability:
The wording of the ability specifically indicates that the deceased ally is being 'revived' and then 'defeated' again. So any abilities that take effect when an ally is revived, or defeated, should trigger in response to this.
This seems to make Mother Talzin the key to unlocking the full potential and synergy of the Nightsister faction's recent rework, and I'd have to assume that, that was intended result
Keep in mind however, that this is only my understanding of how these abilities are intended to function.