How do the Etherealness and Banishment spells interact?

Question

The etherealness spell states:

You remain in the Border Ethereal for the duration or until you use your action to dismiss the spell...

I am aware that the spell does not allow you to go back and forth like the blink spell and others.
Let us assume somebody is under the effects of etherealness. What happens if somebody casts banishment on them?

You attempt to send one creature that you can see within range to another plane of existence

The etherealness spell says you "remain there for the duration" and the banishment spell says it "sends you to another plane of existence". These cannot both occur simultaneously and so I'm left wondering:

Which spell is more specific or which one's description overrules the other? Do you end up staying in the Border Ethereal or are you banished to another plane?

Answer 1

The Creature under the effect of Etherealness should be banished.

I would argue this is the only reasonable ruling. Otherwise, you would be unable to banish any extraplanar creatures that are tied to the current plane they reside on as long as there is an ongoing effect holding them there. I am thinking of spells like summon lesser/greater demon and others.

The rules don't explicitly answer this, however, making the banishment spell fail here would make the banishment spell very difficult to use against the type of threat it is supposed to be powerful against by design.

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Does the creature come back when Banishment is interrupted? - Yes

If the target is native to a different plane of existence than the one you're on, the target is banished with a faint popping noise, returning to its home plane. If the spell ends before 1 minute has passed, the target reappears

Here the rules are fairly simple. The target reappears when the concentration on banishment is dropped prematurely.

Does the creature come back when Banishment finishes its full duration? - No (ambiguous)

If the spell ends before 1 minute has passed, the target reappears in the space it left or in the nearest unoccupied space if that space is occupied. Otherwise, the target doesn't return.

Here there is more ambiguity. Does the effect of the Etherealness kick in after banishment ends or is Etherealness effectively dispelled when the creature is "successfully banished" (the spell lasts for the full duration)?

Again I argue that banishment is a spell that is designed to be powerful against invaders from different planes and making the creature under etherealness just come back would negate one of the main advantages of the banishment spell.

Because of these reasons having banishment suppress etherealness upon completion makes sense from the design standpoint. However, I see a reasonable space for DM to interpret the interaction in the other way. As always it is the DM who interprets the rules of the game.

Answer 2

Spells Don't Cross Planes, Unless Specified

So, banishment cast from the material plane on a character on the ethereal plane would fail.

A traveler on the Ethereal Plane is invisible and utterly silent to someone on the overlapped plane, and solid objects on the overlapped plane don't hamper the movement of a creature in the Border Ethereal. The exceptions are certain magical effects (including anything made of magical force) and living beings. (DMG p. 48)

Spells do Exactly what they Say

If the creature casting banishment is on the Border Ethereal also, then banishment will do what it says:

You attempt to send one creature that you can see within range to another plane of existence. The target must succeed on a Charisma saving throw or be banished. ... If the target is native to a different plane of existence than the one you're on, the target is banished with a faint popping noise, returning to its home plane. If the spell ends before 1 minute has passed, the target reappears in the space it left or in the nearest unoccupied space if that space is occupied. Otherwise, the target doesn't return.

Which because it was cast last, would surpress the effects of the etherealness. If the banishment ends earlier than a minute and the etherealness duration hasn't run out, then the character would return the Border Ethereal plane until the etherealness ends. If the full minute of banishment happens, the character remains banished, as the spell says.

Just Like Summoning

Note, this is no different than if you banished a summoned creature. Take a look at the wording of summon greater demon or conjure woodland beings.

The demon appears in an unoccupied space you can see within range, and the demon disappears when it drops to 0 hit points or when the spell ends.

And

A summoned creature disappears when it drops to 0 hit points or when the spell ends.

Neither mention being banished, and the only way it mentions is dropping to zero or the spell ending; but clearly banishment or wish or planeshift would work to also make them disappear.

The Difference in Language

The difference in language is that etheralness contains the line:

You remain in the Border Ethereal

This is because you are the one going, and you might want to "pop in and out" like a more controlled blink spell; this is saying that isn't what this spell does.