Let's say I copied my rift herald to a watchful sentry when I attached shady spectacles to my watchful, and then also attached Zero Drive. It dies, and then I return units with Zero Drive. What unit comes back?

Watchful Sentry comes back — a 1 [M] Mind unit with Deathknell: Draw 1 — not Rift Herald.

Shady Spectacles only makes the equipped unit a copy "for as long as this is attached to it." The instant the unit is killed, the Spectacles detach (Rule 435), so the copy effect ends there; the card that Zero Drive banishes is simply your Watchful Sentry card. Zero Drive plays the actual cards banished with it, and a card that changes zones is treated as a new object that doesn't carry those layer alterations with it (Rule 124), so nothing about the Rift Herald copy follows it into banishment.

The dying unit's trigger is a different matter: when it died it was a Rift Herald, so its Deathknell ("play a unit from your hand to your base, ignoring its Energy cost") triggers alongside Zero Drive's own Deathknell ("banish me"), and you choose the order they resolve — the Rift Herald Deathknell still resolves using the board information noted before it left (Rule 808.1.d.3, FAQ #1727). But that's a one-shot effect at the moment of death; it doesn't change what card Zero Drive later plays back.