In my opinion it's not only Hayashida's fault but also Yumi's because he just accepted everything that Hayashida did to him. Never complained or said that he was hurting or something and in the end it was Hayashida who told him to leave because he understood that he was in the wrong. They are both in the wrong, but this is only my opinion you are free to think whatever you want.
them meeting up will allow hayashida to comes to terms with his guilt and move on without the regret and yumi will get to see that hayashida is okay now. a big part of yumi not leaving hayashida sooner was because he still loved and cared for hayashida even with the abuse, hayashida had to make yumi leave him after all.
even if yumi is okay now even without meeting hayashida i think it's still okay for them to meet. it'll be like that feeling you get when you come across an old flame from your youth, you moved on a long time ago but it is still nice to see them doing well now. you may have thought you put it all behind you but meeting again brought back some nostalgic feelings. this way they can both let go of their past and move on.
Am I the only one here thinking that the meeting between Yumi and Hayashida is not necessary? I mean, they already knew their problems and they did clean break up, from my point of view.
Even if Hayashida tells Yumi about his problems from his past, I dont think it will change anything. Yumi accepted his behaviour, his abuse to him bcos Yumi knew there was something wrong with Hayashida and the only way to help him was being a sandsack to relieve his anger and anxiety.
Maybe you think that Hayashida still have a regret about his past, not only about Yumi but about everything else. The one who has problem here is only Hayashida. But Yumi already forgave him. And Shuuna loves him unconditionally.
The most important thing is that they are happy with their couple now.