Scrying: an odd interaction with another spell (Spoilers!)

[KorvinStarmast]

The party member cast the scrying spell. They had met the person they were trying to scry on before, but didn't know him well, so the save mod was a simple 0.

Problem.
The person they were scrying on is a Noble who has been a pain in their neck, and they intend to go and deal with him by capturing him and turning him over to another noble.

In case one lf my players is reading (Salt Marsh, Plaid World) I have to put the rest in spoilers.

Spoiler: Kurt Kurageous - and Bron, Velthiss, Laz, Seven Waterfallas, Guy ... Please don't read...

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What the Players do not know, and the PCs do not know, is that about a month ago - after that Duke had royally pissed off the party in the aftermath of the great battle against the Sahuagin lair, a high up NPC serving the Scarlet Brotherhood met with the Duke, presented him with a beautiful gift (a large vase, ornate, expensive) ... and ended up casting Magic Jar on him after he had successfully infiltrated the ducal residence. (Arch Mage NPC, slightly tweaked).

This arcane spell caster (Rosovic) has been impersonating the Duke ever since, and has been sending out Scarlet Brotherhood agents and assassins in various forms. Rosovic's body is hidden under a tub in one of the guest bathrooms of the Ducal residence, the soul of the duke is in the jar/vase, and the soul of Rosovic is in the body of the Duke.

Spoiler: spell mechanics

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Spoiler: the first part worked

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Your body falls into a catatonic state as your soul leaves it and enters the container you used for the spell’s material component. While your soul inhabits the container, you are aware of your surroundings as if you were in the container’s space. You can’t move or use reactions. The only action you can take is to project your soul up to 100 feet out of the container, either returning to your living body (and ending the spell) or attempting to possess a humanoids body.

You can attempt to possess any humanoid within 100 feet of you that you can see (creatures warded by a Protection from Evil and Good or Magic Circle spell can’t be possessed). The target must make a Charisma saving throw.

On a success, the target resists your efforts to possess it, and you can’t attempt to possess it again for 24 hours.

...the duke failed the save, of course.

... your soul moves into the target’s body, and the target’s soul becomes trapped in the container.

Feldrin’s Soul is in the vase/jar.

Once you possess a creature’s body, you control it.

Rosovic has been running around as Duke Feldrin for about a month.

Your game statistics are replaced by the statist⁠ics of the creature, though you retain your alignment and your Intelligence, Wisdom, and Charisma scores. You retain the benefit of your own class features. If the target has any class levels, you can’t use any of its c⁠lass features.
Meanwhile, the possessed creature’s soul can perceive from the container using its own se⁠nses, but it can’t move or take actions at all

. The duke is stuck inside the vase/jar, and it is not transparent. It's dark in there.

While possessing a body, you can use your action to return from the host body to the container if it is within 100 feet of you, returning the host creature’s soul to its body.

Rosovic has not done this, since his various plans have not done the damage that they needed to do before he dumps the mess back into the Duke's lap and leaves, cackling with evil glee if necessary. (or the party stops him...)

Spoiler: what may happen if the party figures this out

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If the host body dies while you’re in it, the creature dies, and you must make a Charisma saving throw against your own spellcasting DC. On a success, you return to the container if it is within 100 feet of you.

Otherwise, you die.

If the container is destroyed or the spell ends, your soul immediately returns to your body. If your body is more than 100 feet away from you or if your body is dead when you attempt to return to it, you die.
If another creature’s soul is in the container when it is destroyed, the creature’s soul returns to its body if the body is alive and within 100 feet. Otherwise, that creature dies.

The Druid cast scrying looking for the Duke; problem I had was will the scrying sensor find the Duke as inhabited by the arcane spell caster, or will it find the body.

1. If the Duke's soul is that the scrying spell would find, then the sensor would be inside the vase/jar ... or maybe it would be outside of it, within ten feet of it?
2. IF the Duke's body is what the scrying spell found, they would see the Duke (who is actually Rosovic) as he goes about his daily shenanigans, plotting and scheming and pretending to be the duke.

I went with the soul, and rolled the dice. Results came up 19/21; even with disadvantage the save was made.
But should I have used the body, and had Rosovic roll the save, not the Duke?
This strikes me a little like casting scrying on something, or someone, who is an illusion of a person you are scrying for.
Similar but different.

How would you have handled this?
Which NPC rolls the save versus scrying?