Yeah, because Toriyama said it but this showed it.
It didn't lead to nothing. I don't know how you can say it lead to nothing.No point in building up courage if it leads to absolutely nothing
It made him more mature than he had been. He grew up, but there's a big difference between those experiences and battle which he wouldn't have been able to get through had he not spent those six months on his own. The filler gives the audience more time with a new character instead of just asking the audience to accept him and be sad for him. It also does a better job at showing Gohan's connection with Piccolo so that when the moment happens it makes a lot of sense and doesn't feel out of place.Gohan will forget about the Robot and the Orphans as they affected nothing.
There was nothing natural about Goku's "progression". He lies about being able to stop an enemy which he has no reason to and never did before, and he doesn't clean up his own mess. That's out of character. At least with Vegeta he was planning on defeating him in the rematch. There's no progression because nothing in Goku's past leads me to think it's a natural direction for his character to go.
This assumes a writer can't make a mistake in characterization. That "reason" is bad writing. Toriyama forgets facts, makes continuity errors, has characters withhold information from other characters just to keep it from the audience even though it makes no sense.I guess you just don't like Goku's natural character progression and you just hate the new Goku as he once again puts everyone at risk to help himself in a fight. Toriyama kept him like this FOR A REASON. Toriyama re-read the manga and still kept him like this. So no it's in character if Toriyama still thinks it is.
Gohan is a five year old and by and large a typical kid at that age. It's perfectly understandable that despite Gohan's training, he would have a moment of crisis. However, he does get over it. You keep acting like that's all Gohan did in the battle. He grew during the fight. "Puss out" is harsh but it's juvenile. Can't you come up with something more mature than that?
Not every character have that Superman Persona. If you notice Goku changes every few arc. He went from killing when someone deserves to be killed to sparing people life to learning that not everyone needs to be spared to seeing everything. Now he just want someone stronger than himself so he can challenge himself. I'm pretty sure Goku is bored deep inside of not having anyone who can rival him that's a fighter(Gohan is a hero. Not a fighter). 
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