NOTE: Personal review. Lots of sarcasm and cheesiness. You've been warned.

Title
Perfectly cheesy. I find it funny how 'cheese' in verb form can also mean exasperate and frustrate someone. Because damn, the cheesiness is frustrating (in a good way) lol.

Description & Foreword
I don't usually like lengthy forewords but this is a strength in your case, since it introduces the interesting premise of this story nicely.

Grammar & Vocabulary
C1: 'Really? To that extend?' - 'Extend' should be the noun 'extent' over here.
Other than that, I don't spot any grammatical errors that are worthy of concern. Vocabulary is pretty much impeccable too, although you do tend to stick to the safe formulas when describing smiles ('smile'/'grin' etc). And since there are so many smiles in this story, they do tend to get repetitive. But frankly speaking, I don't have that much creativity myself to come up with 1000 ways to describe smiles so I probably can't give you advice here xD Maybe stray away from the smiling action and focus more on other facial expressions.

Writing Style
Now I notice that your narration tends to be a tad awkward. It's like too formal and rigid for this cheesy story. A few examples:
C1: 'It was perhaps the millionth time she had heard this kind of cheesy line, but every single time, it still managed to surprise her.' - Sometimes, you can use thoughts to indicate a certain feeling. It doesn't seem like you use thoughts very often, maybe because you use a 3rd POV, but it's okay to reveal some of the main characters' thoughts, especially since it's a more informal and light-hearted story. E.g. 'Seriously? Is there anyone who's immune to my power? (can also be foreshadowing for what is to happen later on) Ah, it is indeed a trouble to have this power...' (italicised since it's a thought)
C1: 'He just seemed so innocent and shy, so this was quite a daring move.' - 'The guy shifted his eyes away from the girl as he fidgeted with his fingers restlessly. (to show that he's indeed innocent and shy) To think that such an innocent and shy guy like him had uttered such cringeworthy words! What a daring move indeed.' Note the sentence structure of the last two sentences. The first one is more dramatic to emphasise the contrast between the guy's appearance and his action, so that the contrast appears more shocking. The second one has a more informal feel as it doesn't follow the typical English sentence structure of 'subject/action/body', and it fits the story's mood better. You don't have to overdo the drama though, because cheesiness + drama is going to make the story way too ridiculous lol. Have a balanced mix, so that the readers won't get too thrown off by what's coming yet the story still retains its unique flavour. The only way to do this is... practice and more practice xDD

Otherwise, I love how absolutely cheesy the dialogue is. The fact that you made so many song references makes the pickup lines even more enjoyable to read (although I was cringing most of the time and screaming 'someone call the doctor I'm overdosed with cheese'). Seriously, I'm appalled by how shameless you are LOL. That's a real charm of the story though, and damn @ the cheese puns in the chapter titles. It gets quite overbearing (since I lack cheesiness in my life /sobs) sometimes but I'm not turned off by it at all. And since there are some 'serious' moments in the story, they help to balance out the cheesiness, though I find the seriousness somewhat awkwardly placed. More to be explained in the Plot section.

Characters
I find it interesting how Hyunmi has a power but is not happy having it at all (well, unless she's getting free stuff out of it). It feels as if her magical ability to bring cheesiness out of people has become the antagonist of her romance story lol (until she met Woohyun that is). Hyunmi, please give me more of your shamelessness. I DEFINITELY NEED IT. T_T At the same time, she's so gullible and trusting I feel like she needs to be protected. Girl, please try and restrict yourself or Woohyun will have to protect you wherever you go xD (Plot twist: Hyunmi is acting like that intentionally for Woohyun >:D) Okay anyway, Hyunmi is a main character that's heaps of fun. I think the insecurity has added a more realistic touch to her character. Even though she's shameless most of the time, she's more insecure when it comes to love because it's not something she is used to having. And with the mission coming in the way... Welp, she was conflicted, and I kinda understand.

I feel so bad for Woohyun- My realistic side is telling me it's biologically impossible to remove cheesiness from someone's body and this whole story is nonsense, but then my crack side is telling me damn you realistic side, stop being a party pooper ;; POOR WOOHYUN. My heart breaks for him. Imagine the trauma he must've experienced!! 3 I don't know if you're a fan of BTS (well, the hope of our life is inside the story so I suppose you are). This kinda reminds me of one BTS crack video where they said: V's story is actually really sad. He was part of a test experiment, and through that experiment, he was given singing pipes, but he also lost his brain cells as a result. /cues sad music. Who are the evil scientists doing this to V and Woohyun?? We have to invade their laboratory with cheesiness! aHEM, anyways, you can totally tell that I didn't read it in an angsty way at all. Okay, back to the point lol. I love how shameless is he too when it comes to himself, because we need someone to stop Hyunmi. Or... make her become more shameless. /guilty look. He's generally a sweetheart. I'm surprised that I'm actually saying this, but between him and Hyunmi, he's the more forgettable one. /shook. Unlike Hyunmi, he hasn't presented himself as a very developed character in terms of his insecurities and doubts. I mean sure, he got dumped by his ex because of his lack of cheesiness, but it didn't seem like he faltered much because of that. And he sure didn't seem like he was worried about his impending death. So compared to Hyunmi whom I felt for, I didn't feel so much for Woohyun although I was fangirling over him. But the good thing is you seem to have written many stories about WooMi (I feel like doing this for a fixed cast of characters to showcase their different facets of personality tbh but sadly I'm not as passionate as you are LOL), so perhaps I can see a different side of him in another story.

I thought GyuHee killed me but WooMi takes it to another level. Their chemistry together is just the bomb. It's like they're legit the perfect couple. /cries. They share that frenemy relationship I love for couples to have, yet they're so unbearably sweet with one another it almost makes me gag. And they copy each other's arguments so much so they can use against the other HAHA. And they both have such lame sense of humour that I feel like facepalming myself. And how Woohyun always makes Hyunmi so flustered. Simply my cup of tea lol. And now I think I know why everybody loves WooMi LOL. Count me aboard the fanclub o/ #woomi4life If I have to be picky, the only thing I wish I can see is depth, as always. Well, Hyunmi and Woohyun only knew each other for 3 days so it makes sense why there isn't depth in their relationship here, but I believe for two people to truly love each other, trust is needed. Hyunmi doesn't trust in Woohyun enough to tell him about her insecurities, and that's a sign they're not quite there yet. But with time, hopefully the two will open up to each other and reveal all their flaws, fears and doubts to only the other party. 

The rest of the characters are fun additions. Special mention to Hyomin though: How exactly did she know when Woohyun was coming back? Stalker alert... I just don't like how she's portrayed as the stereotypical mean girl here. Let's keep mean girl tropes away please.

Plot
Obviously not terribly realistic but who cares? It's enjoyable crack :D But considering how specially cheesy they are, I'm convinced they are indeed made for each other xD Also, I just want to applaud you on how meaningful you've made this story to be. Indeed, learning how to be cheesy is a very important life skill for me to survive in society. This story has impacted me so much by teaching me how to be cheesy. Now I shall spread my cheesiness to everybody out there. //slapped. Okno. Despite my obsession with finding a meaning for every story, I think it's fine this story doesn't have an actual one since it's crack. Unless the importance of being cheesy counts as something.

I actually realised you kinda hinted that Woohyun already regained his cheesiness back, maybe. C1's 'The book he had been studying the past few months seemed useless compared to being around the girl.' He kept saying he doesn't believe her, but then, his narration goes ahead and says this. Is it because he felt Hyunmi's cheesiness himself? HAHA.

The plot twists are pretty great, albeit cheesy (no surprise). I actually guessed that Woohyun might secretly be a cheesy guy and was actually hatching a plot to get closer to Hyunmi, and I was right LOL. Never ever good with guessing plot twists, but yay, I got this one >:D I thought Woohyun made a public confession or something? I mean, they technically kept watching the drama behind the windows, so I'm surprised there wasn't some super dramatic viewer reaction xDD The ending is very cheesy, but also very befitting for the story.

Just a slight nit-pick, but I thought the Woohyun-dying-without-his-cheesiness part was kinda unnecessary lol. It isn't talked about for most of the story anyway so there's not much need to include that melodramatic element.

I like how they only completed 1 thing in the grate list... Eat cheese. Because eating the cheese itself is the only important thing. xD

Flow
Not too bad for the most part. Even though the journey is only 3 days, the characters' interactions are so authentic and fluffy that the duration doesn't seem to matter. The last chapter (the clarification part) feels a little rushed, but it's not that obvious.

Overall Enjoyment
Well, it's been 4 stories and my love for your stories didn't die down ; u ; It's been a while since I giggled over a story like this so kudos to you for making me feel that way. Besides, I have more cheesy lines to use so that's a bonus >:D Although I nitpicked on many things, they didn't really occur to me while I was reading the story for the first time actually (except for the unrealism maybe), so they're just things you can think about to make the story a better piece technically. But in terms of the emotions, you've got it pat-down for the most part.

Just a random thing I'm trying out lol:
Technicality Level (realism, flow, development): 2.5/5 - With flaws, but as expected for a crack story. For this story you may or may not choose to neglect this.
Feels Level (emotions, characters): 4.5/5 - WooMi.
Longevity Level (?? impact and message of plot I guess, relatable characters): 1.5ish/5 - A fun read right off the bat, but unless the readers are feeling incredibly cheesy, I highly doubt it's a story that people will revisit frequently.
Memorability Level (portrayal, twist, overall message, vibes, feels, impact - beginning and ending are key): 4/5 - Cheese.