I think this Mary's story are totally not related to Yumeko u can totally see it in the og where yumekos the mc they dont know mary at all specially her bff is not with too and her personality here are totally diff from the og
I personally think it has something to do with Tsuzura. It's clear in the main series that Tsuzura is gone and all Mary has is her flower pin. It's also clear that Tsuzura's disappearance has something to do with Kirari, hence why Mary hates her guts in the main series. You also see a big change in her personality compared to the main series but what stuck out to me is skinship. Mary doesn't seem to mind Skinship in this series (the past) yet in the main series (the future) she hates it. And It's clear she likes Yumeko (as a friend or more, you can think of either or) but still hates when she tries to hug her. Not to mention she's much more expressive in this series, so another thing that has to do with Tsuzura.
And to answer the last bit of your question which is cheating. In the main series, chapters 71-75 Mary herself said that she'd go as far as to cheat if it secures her win. So she did do it for the fact of humiliating Yumeko on her first day but it was also more for securing her win. But Yumeko, whose skills were a complete mystery, saw right through her and won. After, I'm pretty sure Mary also said it herself but it was also clear that in the Yuriko vs Saotome gamble she was in a state of desperation and needed to get back where belonged, and due to that she lost. While I'm not sure where she exactly picked up on cheating, it's obvious that Tsuzura leaving the school influenced Mary a ton in the future.
well, at the beginning of the main Kakegurui storyline, we know that Mary has achieved a high status and influence over others. And since at that point, everything comes too easy for her, her skills might have become dull, as compared to when she had to fight her way to the top using pure wits
That's because this is Mary's first year at the school and yumeko came second year
After binge reading this series for so long, it’s undoubtedly true that Mary’s godly at gambling?? And this series take place before Yumeko’s transfer but in the beginning of Yumeko’s POV where they played the Rock Paper Scissors gamble, all Mary skills somehow.. dropped if that makes sense. And also she became a cheap bitch and cheated. So idk what happened to her and it makes me anxious, please tell me when won against Aoi I can’t stand that mf