Goku states that he wasnt trying to kill buu, but he probably couldnt have anyways
Goku is able to regain energy etc and is just the same when he returns back alive. He states that the living world has limitations on energy, not that its because he is dead. In otherworld there are no energy restrictions, which is why he gained ssj3 in 7 years and vegeta didnt even come close. He gains absolutely no power when returning to the living world alive
"Ok, look at the fight between super buu and SSJ3 Goku, now compair it to the fight between super buu and Mystic Gohan.......No contest who is stronger there. "
Super buu dosnt fight Goku, Goku runs around and tries not to get hit. He never even harms buu at all. Goku ran from him for like 30 seconds, Gohan went 1-1 with him and actually landed punches and attacks and did a 1-1 ki attack versus buu for around 10 minutes. Goku in no way surpasses Gohan compared to Buu. Goku just runs from attacks. Gohan managed to take the hits and keep dishing it out for quite a while
It was NOT goku's first time to go ssj3, not even close. He tells everyone he did it before, everyone in otherworld knows about his trick, and he does it easily versus Majin Buu
The whole advantage of going Mystic is that you are able to completely control your energy. Imagine it like this...ssj is a match, ssj2 is a fire, ssj3 is a burning building out of control. Gohan loses no energy when he goes Mystic, thats why you dont see all the energy leaking out of him. duh....
Most your nerd knowledge is incorrect
And again, Goku basically admits that Gohan is stronger
Also, Gotenks ( a fused warrior) gone ssj3 admits that Gohan is far stronger than him. They ADMIT that ssj3 is a weaker level than mystic