I think you meant volume 12, volume 14 is all about the Semi and Final though it still took place on the same day.
Spoiler for Volume 12:
The problem is that it was not Arata's team. Arata only entered single tournament the next day and was only there a day earlier to cheer and watch Chihaya. The Fukui West team Arata was in was actually from a different school. They won the Fukai representation despite have only 3 members.
The problem they had was that one of the member can't make it in time due to traffic and the team leader was Arata's middle school friend who appeared in one of the Arata's flashback. (the one who tell Arata that he is now a A-Rank player)
They know they can't win with only 2 people and will be force to forfeit, but they don't want to forfeit without even playing a match so they ask Arata to stand in and act as dummy. This way the two of them can play even though the team will still lose. Arata reluctantly agrees and stayed put until one of the other member lost which guaranteeing Fukai team's elimination. At that point, seeing how they really like Kurata but wan't able to play to their potential due to missing teammate, Arata hope to lift their spirit and start to play with everything he got eventually coming back from -15 disadvantage and during that span, lifted the spirit of the remaining Fukai West member and end the match with 2 wins to prevent the shutout.
However Arata's play stood out too much and the leader of the local club he join in Fukai (who was there as a organizing committee member) recognizes him despite the mask and the issue was brought to the committee. One member of organizing committee initially wanted to ban Arata form the singles competition the next day but other argued that Fukai West didn't win and that Arata waited until his play was of no consequence. Others also argues that Arata was a special player and perhaps the only realistic hope to take down the current Meijin Suo, who was hated in the Kurata community due to his antics. In the end the committee decided to allow him to participate albeit with some disciplinary action, but before they can tell him that, Arata took himself out of Singles due to his guilt and sense of responsibility
As Arata was leaving he met Shinobu who comes back due to postponed Birdman Competition. (one of her hobbies) After hearing what happened she tells Arata to stop being cool and drag him to the committee to apologize and tell committee that if Arata doesn't play then she'll withdrew as well and force Arata to apologizes and ask committee to allow him to play the next day. Arata was still reluctant but Shinobu asks him that doesn't he want to play her, and that she already disappointed once by today (Birdman Competition) and she doesn't planned on being disappointed twice.
Since the committee wasn't plan on pulling Arata anyway, they agree on the condition Arata and Fukai West team each needs to write an apology letter and that Arata is forbid from entering the competition hall that day to watch and cheer his friends.
As for Shinobu's obsession with Arata, it's simple. She is the same age as Arata, Chiaya, Nishida and has being playing since elementary years like Nishida and Arata. Since Arata has won the national all 6 years he was in elementary that means that not once had Shinobu won any national title in her younger days despite being an A-Rank player since 4th grade. Which is why when Suo asked Shinobu for a match and asked her doesn't she want to play him, it was young Arata's face that showed up.
BTW, Taichi is still a B-Rank player and can't enter A-Rank National tournament right now unless he won his B-Rank national which would allow him to move up to A-Rank and enter the coming up Meijin Prelim.