The reason why I came back to Tohno's mansion is only one, my father was very annoying about it.
Since my father is no more and so are my duties as the successor, until I graduate the me which has been elected as the proxy, will do as I please since I have no reason to come back to this mansion. At any rate after I graduate, I will always stay here.
Is it correct or am I wrong?
The fan translation is:
The only reason I came back to the mansion was because my father wanted me to. Now that father is dead and I do not have the responsibility as the successor until I graduate, there is no reason for me to come back to this mansion. Because when I graduate, I will live in this mansion for the rest of my life.
I am not sure about this passage:
亡くなり refers to the father and I believe it also refer to the duties as the heir. With 卒業 starts another different sentence. That's how I understand it.
So I would say