Does an inverted Magic Circle spell prevent a conjured creature from vanishing at the end of its spell's duration?

Question

The Magic Circle spell (which takes a minute to cast but lasts an hour) can be used in an inverted way:

When you cast this spell, you can elect to cause its magic to operate in the reverse direction, preventing a creature of the specified type from leaving the cylinder and protecting targets outside it.

Many planar entity conjuration spells, such as Conjure Woodland Beings, last up to exactly an hour, at which point the creature vanishes:

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

Imagine the following scenario: Bob the Druid casts Conjure Woodland Beings, which makes DM-decided fey creatures (let's say 2 quicklings) appear, and maintains concentration on it. After 30 minutes, Dylan the Wizard decides to cast an inverted Magic Circle around one of the feys, which Bob orders to stand still. After that, Bob drops concentration on Conjure Woodland Beings. What I'm wondering here is what takes priority between "disappears when the spell ends" and "preventing a creature from leaving the cylinder".

Does an inverted Magic Circle prevent a Conjured Creature from vanishing at the end of its spell's duration?

If so, that means that the now-trapped fey is no longer friendly to / controlled by Bob, but remains there until the Magic Circle itself ends (total of 1 hour 31 minutes). If not, that means that the fey disappears as soon as Bob's concentration ends, regardless of the Magic Circle (total of 31 minutes).

Answer 1

No

The reversed Magic Circle needs to be read in context. In particular:

The creature can't willingly enter [leave] the cylinder by nonmagical means.

The creature is not willingly leaving so the spell does not prevent it.

Answer 2

Nope!

You can't use Magic Circle whilst maintaining Concentration on another spell:

• Conjure Woodland Beings: Duration: Concentration, up to 1 hour
• Magic Circle: Casting Time: 1 minute

The Player's Handbook on page 202 reads:

Longer Casting Times

Certain spells (including spells cast as rituals) require more time to cast: minutes or even hours. When you cast a spell with a casting time longer than a single action or reaction, you must spend your action each turn casting the spell, and you must maintain your concentration while you do so (see "Concentration" below). If your concentration is broken, the spell fails, but you don't expend a spell slot. If you want to try casting the spell again, you must start over.

You resolved this using multiple casters, however you forgot they must be willingly leaving, Conjure Woodland Beings, along with any Conjuration/Summon spell I could find (official or UA) doesn't send them back from the creature's choice, the magic from the caster was what summoned them and maintained their presence.

Your conjuration spell would require something not suffering from “The creature disappears … when the spell ends.” I think WotC managed to prevent you from reaching this using their recent rules updates, perhaps using older 5e rules you could do this using a Celestial or Demon, but then again why would you want one of those angry at you?

Answer 3

Depends on whether they pass their saving throw

Consider this part of the spell description:

The creature can’t willingly enter the cylinder by nonmagical means. If the creature tries to use teleportation or interplanar travel to do so, it must first succeed on a Charisma saving throw.

If the spell is reversed then that should still apply in reverse. The creature can't leave the circle when the summoning spell runs out unless it makes a Charisma save.