
The first cover had all three of them since the beginning, you can check the studio account where they have the first cover.
2nd, the reason many thought Cain was endgame was with Jooin monologues esp when Cain first left. Talking about how he miss him and the warmth he gave, then talking about how he wanted to embrace Cain the way Cain did for him. The tip of the ice was the ring. So many really believed there was a change of direction that Cain was gonna be endgame.
Even during the birthday chapter Jooin said he liked Cain and does want try things with Cain. So all this collaborates into why others thought he was an option, not necessarily the cover you know.
i dropped this long time ago bc i know he'll end up with yahwi anyway. what's funny is that readers still tried to reason out that cain will most likely be the endgame— the first cover of this manga was only jooin and yahwi, that was already your biggest sign, why did people think cain was an option??? he was there for the plot development (if there actually is)