Look, you can't justify Mystic Gohan. At all. It was the absolute worst moment in the whole series. Spend the entire series hyping up Gohan as being a hidden badass, have basically the supreme god of the universe personally bring out his hidden poential, then toss him under a bus.
Why?
Because apparently it's better if Goku kills Buu using a spirit bomb instead.
Gohan got depowered. He beat Cell with SSJ2 because Goku
let him do it (more like made him do it), because Goku wanted to give up the hero business, and he was fine with dumping that responsibility on his kid. But then Gohan went back to living a normal life after Cell.
While fighting Babidi's goons, Vegeta complained with disgust that Gohan was weaker than he was when he fought Cell.
Meanwhile, Goten and Trunks were capable of going SSJ3 while fused, and are an apparent match for whichever stage of Buu that was.
Old Kai apparently brings Gohan back into the fight by unlocking ALL of his power, leaving it all at his easy disposal without the need for transformations, which makes Gohan about equal to Buu after he absorbed SSJ3 Gotenks. Which means that Mystic Gohan's power is ~ 2x SSJ3 Gotenks. But Mystic Gohan lost the fight because Piccolo's mind gave Buu the edge.
SSJ3 Goku arrives with the intent to beat Buu by permanently fusing with Gohan, and Goku's SSJ3 power is about even with Buu, meaning 2x SSJ3 Gotenks, same as Mystic Gohan.
But then Buu's SSJ3 Gotenks power expires, which relieves Goku because he doesn't have to perma-fuse with his son, and lulls Goku into a false sense of security, which Buu exploits to absorb Mystic Gohan. Which sends Goku off to permanently join Vegeta as Vegito (forever doesn't last very long).
Vegito uses his advantage to save his friends from Buu's absorption.
The real throwing Gohan under the bus comes from Gohan not taking part in the next (and final) battle after being rescued, but it's possible that Buu's anti-magic innards undid Old Kai's magical unlocking, returning Gohan to SSJ2. From a story standpoint, I think I'm okay with that, because it says that Gohan is potentially every bit as strong as Goku, but Goku's about to be retired, and now the future belongs to Gohan, and he has room to grow.
It's DB GT that messed the rest of the story up for Gohan.
