The entire arc is just full of conveniences and poor decisions made by Harlequin over and over so he fails in protecting the drug. It's to be expected when the sins are so strong to the point of only Arthur being a real challenge to any of them individually. Just look at the Liones arc when Meliodas was hindered by Tristan, and Elizabeth was just written out of the picture entirely. This will keep happening in order for the plot to progress. It's ironically what happened to the SD/DK when the sins had to fight them. The only sin who had a good result was when it didn't matter to the plot (Ban).
I'm sure Diane will get there and be blackmailed to hand the drug over for Sixtus life or something, since we need to force every sin to be nerfed directly for Camelot to ever be a threat to Brittannia.
Now that you've said it, that really is the problem with this arc. None of this would've happened without the forced conveniences that happened back to back to back.
Let's have a look:
- Worreldane and Killbegan just happened to stumble on a way to enter the Fairy Realm. If Myrtle wasn't there, what then? They just poison and kill every fairy who comes out of the gate until Harlequin turns up? Okay? And what then? You'd be daft to think he'd just give you the drug or you could force it from him. Then he goes back to the fairy realm and places the world on lock-down. Missions over.
- W & K only had one distraction planned for Harlequin, but not for Diane until Myrtle. How did W plan to take the drug if she had Diane and or Harlequin chasing after her? Even if Killbegan seals the magic of one of them, the other can still deal with W and force her out.
- W goes in blind and just happens to have the spell needed to trick the system to give her the drug. Okay, ignoring that. What if the drug wasn't there? What if Harlequin kept it on his person, or if Nasiens had it? What then? They don't even ask Harlequin for the drug, Worreldane just immediately dips to the throne location as if she already knows lol.
- W & K get lucky when Harlequin takes the clear and obvious bait instead of the safer choice.
- W & K get lucky again when Harlequin has to use one of his forms to keep Nasiens safe (never happen if Myrtle wasn't there), which weakens his consecutive spirit spears. Killbegan would've probably been one-shot had the sunflower been the only active spear - since we know from Q&A, each consecutive form ''greatly lowers the power and precision'' of King's spirit spears.
- W & K get lucky again that Harlequin doesn't kill Killbegan before Sixtus is put to deaths door or W has enough time to take the drug. Since if he felt that with no Killbegan breathing down his neck, he would've blitzed over to kill W.
- W & K get lucky again that Diane chooses to go help (why?) Harlequin instead of her children, when she just saw 3 of them be hurt and has more incentive to go help them.
- W & K get lucky again that Sixtus gets puts to deaths door, just as Killbegan is within arm's reach of Harlequin so he gets distracted, off-guarded and underestimates Killbegan's trick.
Like oh my god, the amount of convenience in this arc LMAO? Literally King/Diane making every bad choice in the book possible, and their current success relies entirely on Myrtle's existence which wasn't a planned factor. If one moment listed above had either of them taking the safest choice possible (i.e not risking their children's lives when they just saw 3 of them get badly hurt) then the plot would've been resolved instantly.
Like just imagine you're Worreldane here.
''Hey Worreldane. Can you go steal the drug of yore for me? It's in the fairy realm, which we don't really know how to get into, and once we do, it's under the protection of two members of the seven deadly sins. You know, the people I told you guys to avoid fighting actively? Yeah, thanks x'' - Arthur to Worreldane, probably.
And then Worreldane to Killbegan: ''Hey can you distract two members of the seven deadly sins and curse the Fairy King who will never fall for your trick unless he's practically not respecting you at all? You're asking me how you will 1v1 the Fairy King, and survive long enough to curse him? I don't know, figure it out yourself? Whilst I go find a drug which for all I know, could be on the Fairy King's person or locked away for good. Yeah, thanks x''
At least this arc does very well in building Kiane's family and the eventual return of Percival, because jesus christ is the conflict just ''let camelot win'' at this point.
