Possible crack-theory/ food for thought: How likely would it be for real! Ciel's soul to "be stuck"/ "hiding" in the Phantomhive ring and communicating with our! Ciel from there? The "earl" said once, he can still hear the whispers of the former Watch dogs. The scene of signing the contract was drawn like three times, adding more cruel details in the respective arcs - yet not one panel of Sebastian devouring the boy's soul (despite not shying away from "jokes" of eating "his master"). That idea would not clash with Ciel's state; the blood transfusions could keep him "civil"/ "sane" enough not to attack his brother. However, when it comes to the Bizarre Doll's first appearance in Campania, arguments might get weak. Though, it could be "traces of Sebastian's last meal", they are sensing. In the Circus Arc, Will observes the demon butler's behavior and concludes how he "must feel" from there. Shinigami were surprised by Sebastian's sight, so he must have "walked the Earth" before their rebirth. 100+ years ago. His forms after the ritual confirm. But: That does not mean, he took a break from hunting or concentrated solely on Britain. Especially with the amount of cults around. Open question so far, did they intend to summon one demon in particular or did they get a "random" answer? With how the concept of Shinigami seem to work, you'd expect to hear more about multiverse. If that fanon becomes canon... Shinigami won't be the only ones capable of time traveling.
Where is real Ciel's soul?
That seems to be the first question here. I'm a bit confused by this ask, but I'll answer what I can.
No, it's not in the ring. Even though we don't directly witness the demon devouring real Ciel's soul, it did. Sebastian says he's already taken/claimed/devoured real Ciel's soul as payment for "crossing" from his world into the human realm. When our earl later asks Sebastian what happens to souls he eats, the demon says "nothing" but then says they "disappear" by the time Judgement Day comes along. What I'm saying comes from ch139:
So, theoretically, real Ciel's soul could still be somewhere inside Sebastian's demonic guts, just like the ring was still inside real Ciel's guts. And it's just sitting there until it eventually disappears. It's unclear just how souls provide energy or sustenance for demons, but I theorize the souls they eat do have an effect on them. So, I do think that when this demon devoured real Ciel's soul and then took a human form, that form was partly based on what real Ciel (and our earl) would have looked like as an adult. This helps explain why Sebastian and Vincent have visible similarities. It's like Sebastian (in his current human form) is a combination of Vincent, Rachel and something purely demonic. The demonic part accounts for the pitch black hair, black nails, and eyes that change from human to demonic. He apparently cannot change his hair color or nail color. It also might be impossible for Sebastian to make his eyes look blue, like the twins' eyes. If you gave Sebastian the signature Phantomhive hair color with real Ciel's side part, changed his eyes to deep blue, and made his nails look normal, this is basically what a grown-up real Ciel might look like. And grown up "our earl" would just be the mirror image of that.
Real Ciel's soul also seems to affect the demon's behavior, as there are things Sebastian does... from time to time... that remind me of real Ciel, like: 1. Offering our earl warm milk and honey when he was at a loss what other food to offer and didn't want for his young master to go to bed hungry and 2. Trying to spoon feed our earl while he was sick; real Ciel also did that once when our earl was ill.
What was the demon doing before this contract?
That seems to be another question in your ask.
Apparently sleeping! Sebastian wrote the lyrics for the Phantom Five's songs, and one is from his perspective as a hungry demon, awakened by the aroma of a soul. He says sleep is a luxury for demons, and so he might have been taking a long pleasure nap after a previous meal.
When was that previous meal, you might ask? Probably back in the late 1700's, judging from two things: 1. One of the outfits he offers to wear while serving his master is from that time period, and 2. He says he danced the Viennese waltz at Schönbrunn Palace, and that dance didn't exist until the second half of the 18th century. He might have had a contract with a member of the Hapsburg family in Austria. Either that or with someone in their social circles. So, Sebastian definitely isn't a demon who only offers contracts in Great Britain.
If the third outfit he appears in is any indication, he's also had a contract dating back to the late 15th century, possibly in Italy. For whatever reason, the app isn't letting me paste the link I grabbed for an old post about this third outfit, but if you search my blog (or click the tag on this original post) for "ch138", it should be one of the posts that comes up; it's dated August 26, 2021.
What demon was the cult trying to summon?
I'm not entirely sure. The cultist calls the demon by a number of descriptions or sobriquets, with the licensed English translation saying "the devil", "lord of chaos", as well as many wordier phrases. From ch136:
Interestingly enough, one of those wordy phrases is "the nefarious beast who sups on spilled blood", and that might be what is happening in this image:
If that's the case, then the soul might have spilled out with the blood, and this image would be of the demon devouring real Ciel's soul in order to cross over into the human realm.