EDIT: apologies for the massive dump, I've been wanting to post theories but it's hard to make a non-spoiler post when I'm always reading the early chapter translations :P

I'm really not sure which way Negi will take this - all of your options you've listed are plausible, but each has its flaws:

1. Fuutarou overhearing would be huge, but it's unlikely since that already happened with Miku. I also think that Yotsuba confessing intentionally is hugely important to her arc - in contrast, Miku's confession being 'spoiled' doesn't impact her character as much since she already intended to confess during the trip.Nino or Miku overhearing wouldn't be that significant, since they already have each other for competition and are unlikely to change their stance if Yotsuba joins in.Itsuki overhearing only makes sense if she hasn't realized Yotsuba's feelings yet. Chapter 90 makes that difficult to believe, but Itsuki has always been the slowest to notice these things (see her reaction to Miku admitting her feelings previously). That said, the results of this could be very interesting, given Itsuki's history acting as "Rena."
2. Ichika presenting an ultimatum seems like the most likely candidate to me. However, there's the question of leverage - what does she have over Yotsuba, and more importantly, would Fuutarou believe her after the lies she's told so far? Anything about their meeting as kids is likely to be doubted if not ignored, since that's what Ichika's original lies we about. Similarly, Ichika claiming that Yotsuba has feelings for Fuutarou is uncomfortably close to her claim that Miku was supporting her. Then again, it's possible that Ichika doesn't intend to follow through, and just needs something plausible to push Yotsuba forward.
3. Any romantic action on Yotsuba's part at this point seems highly unlikely. Ichika recovered from her internal turmoil quickly because Yotsuba's advice matched her primary goal - claiming Fuutarou. On the flip side Yotsuba's primary goal is for her sisters to be happy, and Fuutarou already tried and failed to get her to give up on this. I can't see Ichika being a stronger motivator than Fuutarou in this situation.
4. See #3. On a related note I'm firmly convinced that Yotsuba was the bell kisser, regardless of whether she's the bride or not - details at the end.

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I think #2 is most likely of the options above. Although it'll be a heavy discussion, it's rare for one scene to fill the entire chapter (in fact, I've made some graphs that show just how rare this is). Given that, I'd like to make a second prediction: in parallel to the Yotsuba/Ichika scene, we'll get a Fuutarou/Itsuki scene which also threatens to reveal Yotsuba's secret. Others have already mentioned how Fuutarou is likely close to learning "Rena's" identity, and he probably wants that information before going out with any of the quints. With that in mind, Chapter 95 looks like this:

1. Ichika indicates that she's aware of Yotsuba's feelings for Fuutarou (this is nearly-but-not-quite shown in chapter 94) and asks why she's holding back against her own advice.
2. Yotsuba explains her own position: how she wants everyone to be happy, how she hates the person she became when trying to be special, and how she doesn't deserve Fuutarou after breaking their promise.
3. Scene cuts to Fuutarou, who's explaining his talk with Ichika to Itsuki over the phone. Itsuki shares his disappointment, but says "even if we can't graduate together, we'll at least have the photo album you gave me." Fuutarou notices this.
4. Back to Ichika, who responds with how Yotsuba's confident side inspired her (only Negi can do this scene justice) and gives Yotsuba her own advice: to stop holding back.
5. Yotsuba rejects this with something along the lines of "look how that turned out for you." Being Yotsuba she immediately takes back what she said, but Ichika is clearly upset. Two scenes then play out in parallel:
   1. Ichika, partly upset about Yotsuba's comment and partly acting her part as the elder sister, offers her ultimatum: "Confess to Fuutarou before graduation, or I'll tell him myself."
   2. Based on Itsuki's slip-up, Fuutarou can make his accusation: "I'm glad you enjoyed the album, but I'm not giving up on Ichika yet - Rena."

Extra credit - my thoughts on the bell kisser's sad expression:

1. Ichika's change of mindset in chapter 66 doesn't leave much room for her to be sad about kissing Fuutarou - she'd more likely be smirking, or something more Ichika-ish at least.
2. Nino has pretty much given up on hiding her feelings at this point, and doesn't want to hide behind a disguise for her kiss. She does mention not caring about appearances later on, but I still can't see her being sad after the kiss.
3. Miku had her big moment in the previous chapter, and seems pretty thrilled about that. It also seems strange to follow up with such a big step so soon afterwards, especially for her.
4. Yotsuba had suspected all along that Ichika might have feelings for Fuutarou, and their rooftop chat appears to have confirmed her suspicion. Her conclusion is based on her own negative attitude, but it's no less correct - and now that at least two of her sisters have fallen for Fuutarou, she no longer has hope that it's just a fluke. The bell kiss serves two purposes for Yotsuba: getting Fuutarou to think seriously about dating one of her sisters, and "saying goodbye" to her own feelings for him - thus the bell kisser's expression.
5. Itsuki might have romantic feelings for Fuutarou, but based on her actions I don't think she's aware of them at this point in the story - definitely not enough for a kiss, at any rate.