This is the closest thing I could find to Goku allegedly holding back:
Chapter: 46, P12.1
Kame-sennin: “I won’t hold back.”
Goku: “Me neither!”
That doesn't really mean Goku was holding back on Nam.
Yeah but they are still more powerful than it. They might be wounded but they aren't getting killed by it. And Initial Nappa just tanks it since Scatter Shot >> SBC.
I am not sure it's all Toei's fault. Toriyama did have a hand in some of Super's scripting, including some of the anime's. It's a retcon just like how Goku absorbing SSJG I'd.
Gomah turned them into kids after witnessing what they were capable of in the Boo arc. Surely the narrative intent was for them to be weaker because that's the version of them Gomah feared.
No SSJ2 was used to beat Tamagami #3. That just means SSJ2 Minis are stronger than Pre Majin Dabura. Obviously Goku doesn't know how strong the Majin boost made Daburs though so in his mind the Tamagami should be stronger than Majin Dabura but Goku still needed SSJ2 to win.
Goku being at 90% like you have him seems about right to me. Initial Goku should have an advantage over Metallic. Goku would have won without the KHH if Metallic wasn't a robot.