Already possessed walking into a Magic Circle?

[Avigor]

1: What happens? Hallow specifies the effect ends, Protection Against Evil and Good specifies that the subject gets advantage on any additional saves after receiving the buff, but Magic Circle seems to have forgotten to specify what happens to existing possessions (or charms, or frightened conditions).
2: Would it make any difference if the possession was from one of the specified creature types or from a humanoid wizard having cast Magic Jar before walking the possessed person into the Magic Circle?

[90sMusic]

I've always made the effect end when you enter.

It says "targets within the cylinder can't be charmed, frightened, or possessed by the creature"

Seems pretty cut and dry to me, if you're already afraid of it, doesn't really matter because once you enter that circle you can no longer be frightened by it. If you are already possessed, when you enter that circle, you can no longer be possessed by it. And so on. I read it as conditions rather than actions that cause those conditions. If you can't be frightened by something anymore, stands to reason any fear you previously had of it would immediately cease.

Some DMs (and I know a lot on these forums are jerks that play against their players in this manner) would try to wordplay it out to not work that way, but this is how I do it.

And as you already stated, similar spells and effects explicitly state it ends those effects. Perfectly reasonable that this one does too. No real reason this would be the odd man out.

[Seclora]

I have always understood that a creature currently possessed would not enter a Magic Circle, since the controlling entity cannot choose to enter it. If it did enter by accident, it would be either trapped or forbidden, depending on the facing of the circle. The spell then acts as Protection from Good and Evil, since that is a lower level spell, and not like Hallow, a much stronger one.

Magic Circle and Possession from Good and Evil say nothing about protecting from Humanoids, and would do nothing about a Wizard using Magic Jar. Which is stupid, because it makes Magic Jar really hard to stop.