I would like to know how much Kazusa loves Haruki in both the anime and visual novel. I feel bad because she took a plane and left.
Yes, Kazusa loves Haruki deeply.
Before answering the question I must say I would recommend reading my other answer to Setsuna's love for Haruki as it helps understanding this answer better (the game is a love triangle after all):
Regarding your question about why she runs away it is because of Haruki's indeciveness. Even so she tells him not to say anything to Setsuna about them sleeping together because she wants him to be happy with her. She also says in the visual she says she hates betrayals so she hates herself.
After having read the visual novel some of the resons that prove Haruki is the person Touma loves the most are:
She'll lie, cheat, destroy anything and betray anybody. Kasuza would gladly never play music again if it meant that she could have Haruki. In Coda Kazusa shows she would gladly destroy her hand [ending her career as a pianist] if it meant having Haruki.
Once Kazusa left Japan in Introductory Chapter Setsuna told Haruki she could be Touma's replacement. After desperately pursuing him for 3 years and crying every time she failed Setsuna finally gets to start dating him. The next 2 years with him made her become as happy as she had ever been. Once those 2 years have passed Kazusa returns to Japan. After lots of suffering without Haruki she's more determined this time and in her route Kazusa ruins Setsuna's life by proxy by forcing her fiancee Haruki to break up with her and telling him to move with her to Europe.
She wants Haruki to be happy: In introductory chapter after Haruki enters an official relationship with Setsuna, Kazusa pretends to be happy so as not to worry him. Haruki's late confession to Kazusa makes her reveal that having Haruki at hand yet never be able to have him is a nightmare for her. In Coda Kazusa admits she can never fall in love with someone other than Haruki, that she will always love him and that having him is her dream.
In the game if Haruki 2-times both girls then Kazusa runs away because only Setsuna can heal his heart:
No! You'll become the most miserable!; Only she can heal you, I cannot.
On the other hand, if he properly breaks up with Setsuna and he will have no scars then both of them get married and alienate from Setsuna. Kazusa says:
I have become the happiest girl in the world; It is the best thing that ever happened in my life. Truly
At the same time she also says that even though it was at the expense of causing her friend to become the "most miserable girl in the world" (obviously Setsuna) she still feels tremendous happiness.
Touma loves Haruki, that's a clear fact. As a matter of fact more than any other person. She herself says in Coda she can't love someone else.
In Introductory Chapter Setsuna says: "Kazusa has eyes just for you"
In Introductory Chapter Kazusa says to Haruki: "I can never have you, yet you want me to stay around you forever? This nightmare was your idea?"