“Also, is there ever an event where a gandhabba is completely annihilated, such as in a parinibbana of a Buddha?”
Yes, at parinibbana of a buddha or arahant, the gandhabba is no more.
Lal often gives the analogy that the gandhabba is like a heater coil. Inside the body of an arahant it is fine (like the coil immersed in water); outside the arahant’s body it cannot “bear” it (like the coil burns outside of water).
In this old forum, you can see Lal’s answer that is related to your question:
“So, when the physical body of the Arahant dies, the gandhabba comes out and perished.”
Also, you may find the following insightful: we cannot talk about a gandhabba of an arahant who is in nirodha samapatti either.
Nirodha samapatti is like parinibbana, except that the physical body is still alive.
See #13744, (5) of this forum: