I don't quite agree on your theory on why she dropped it, to me it seemed more like she dropped it because it wasn't refined. It lies in the values behind the game with the poems and tradition like Kana mentions when she joined the karuta club. Since then the sports evolved into a far more competitive one especially if we compare it to how Kana perceived karuta in her imagination to reality. Basically I believe she dropped her way of playing karuta because it was unladylike and as queen she felt she needed to be as refined and ladylike as possible. Consequently during the match against Chihaya she figured why would does it matter if she's not all proper and ladylike.
To be fair it's only my theory but you have to ask yourself is there really any guilt there? Why would there be any guilt involved when she's right and honest? Do you feel guilty when you speak up when you know you're in the right and play fair? Why would it be unfitting for the queen to do it? I'm sorry but I can't see any logical connection between guilt and her dropping her way of playing.
If my theory is right you're basically knocking her for being unladylike yet you've praised Chihaya for being unladylike and honest with herself, it might be worth thinking about it.
You don't have to rewatch it but buffalobiian gave you a timestamp for the second card she called Chihaya out on and as for the ones I took a screenshot of it was at 17:01 so you only need to check those moments.
Most of the time it is clear who got the card first but there will always be times when it's questionable, Chihaya had a moment like that with Shinobu the difference then was that Chihaya decided on her own that Shinobu should have the card even though Shinobu offered the card to her first. Does that mean it's both Shinobu and Chihayas fault for not making it clear? Is it possible that it could have lead to an argument about who touched it first? Same thing with Sakura in ep 18 if Chihaya sent the card flying instead what would have happened? Is Komano corrupt for the methods he employs? Besides from what we've seen Yumi only did this twice both of which we can confirm/check that she did touch the card first. Also I've stated why it's more possible for close calls with Chihaya considering how fast she is.
So her being in low spirits from as Nishida speculates being unable to live up to all the expectations automatically means she's likely to start cheating? She's hardly done anything wrong. Ok she held up the games for everyone else but if we go back to what I described with Ririka do you really think it's more important for her to not speak up when she truly believes she's in the right? What are you suggesting she should do?
So far it looks more like speculations for why she is a scumbag but nothing definite. Some good points she has, judging from the marks on her hands she practices karuta a lot and gives it a lot of effort, she's shown gratitude to those that guided her and she's honest and speaks up for what she believes in.