In a fiction first game like this, there are always going to be lots of different ways to bring mechanics to bear on a cool story element.
I think it’s important to remember that in Blades, things happen to PCs in 3 ways: the GM simply says they occur, an action roll calls for a consequence, or a devil’s bargain creates an effect. There is the unusual fourth possibility of players causing negative effects to one another, covered in the PVP section (only if all players involved agree to terms).
You could say, “Every time the charm is pulled out, X happens to the Tycherosi.” You can say that’s an honest depiction of the world, and your breakdown of the potency factor would be one way to figure out the strength of that reaction. This would work much like the mechanic for supernatural fear in the game.
Personally, though, I think Blades shines when we don’t focus on things that happen all the time, but rather when we zoom in on them at the specific moment they become interesting and find out why that instance is special.
I’d rather come at this question from a place where we note and discuss the fictional discomfort caused by the charm as a base point, and then see if that flares up into a dangerous (mechanically represented) problem at important moments – largely through consequences and devil’s bargains.
Once the charm appears, you can describe the initial effects however you want (“The demon pulls up short, pausing warily for a moment to sniff at the strength of your charm instead of simply tearing off your head. The Tycherosi stiffens as well, taken aback by its baleful presence.”).
This puts the ball momentarily in the PCs court. They have a chance to act, instead of reacting to the demon’s intention. Do they try to escape? Consort with the demon? Fight it? Whatever they do is going to be dangerous (likely Desperate/No effect to begin with).
This is when I might use the charm’s potential effects in a devil’s bargain (“You can have an extra die on your attempt to escape, but in the panicked rush to get away the charm gets too close to your friend and they’ll have to deal with the results. Do you accept?”).
You can also introduce effects if the players’ actions result in consequences (“Okay, you attune, trying to amplify the charm’s effects to drive away the demon. A 5? Awesome – you supercharge the charm with energy from the ghost field, and the demon howls with anger and dematerializes. He’s gone. However, the charm has also affected your Tycherosi companion. He’s gone wild-eyed and psychotic – he ‘won’t stop till they’re all dead’ and ‘can’t tell friend from foe.’ Want to resist either of those?”)
As for the level of the danger posed by the charm, I’d mix it into (or base it on) the situational position. Rather than just considering the potency of the charm in a vacuum, I’d want to consider how much trouble it can add to the confluence of factors in the scene. How fraught is this moment? Does the charm make it worse, or do they skate by it this time? An overall desperate situation could absolutely make the Tycherosi’s reaction more extreme.
In the case of a devil’s bargain, I usually lean away from giving one player a bargain that would put an irrevocable consequence on another player’s character. If I were to offer the whisper in this example a bargain based on the charm, I’d make it something like, “the Tycherosi will have to deal with the charm’s effects.” Then, that struggle is inevitable – but perhaps the consequences of the struggle can be resisted. (This is a fine line, but I’d usually favor it.) If I offered a bargain to the Tycherosi in the same scene, I’d be more up front. “You get an extra die on this test, but then you take Level 2 Harm “Nauseous” because you’re so close to the whisper.” No resisting that one.
By the way, I think the clock idea would be great. A dark intention building up over time due to exposure to your ally’s magic item is pretty good stuff.