1. In a ki beam struggle, the beams cancel each other out to an extent. Goku didn't just hit Piccolo with a KHH, he had to go through Piccolo's beam to do it. Another good example of this is Goku vs Vegeta. This has interesting implications on Gohan vs Cell if you accept it, but amps are so much bigger by that point and that fact alone is probably enough to explain away what happens there.
The author didn't spend a lot of time doling out some of these assigned power levels. There are severe repercussions to his suggested ki amplifications. Let me ask you a series of questions which will show you the issues which has forced me to alter the author's original work (for the better) to make a better, more cohesive in universe world.
Imagine if Goku and Vegeta were of equal power in the Saiyan Saga.
Goku 18,000
Vegeta 18,000
What is Galick Gun and what is Kamehameha?
Now imagine Goku vs Vegeta equal power levels on Namek.
Goku 3,000,000
Vegeta 3,000,000
What is Final Burst Cannon and what is Kamehameha?
Ok, now imagine Goku vs Vegeta at equal power level at the Androids Saga.
Super Saiyan Goku 250,000,000
Super Saiyan Vegeta 250,000,000
What is Big Bang Attack and what is Kamehameha?
See, the thing is, the author has a narrative. He ambushed himself with his own numbers and his numbers destroy his own narrative. We can only make one choice, we either stay with the numbers and get a false narrative or, we stay with the narrative and change the numbers. That's the best we can do. For me, I believe the in universe narrative is far more important to the random numbers assigned by a forgetful author who didn't spend more than a couple of hours at best to come up with them. We must stick with what is logical. If the author was able to redo all of his power levels, he surely would change them. Unless you think he got it perfect within a couple of hours of thought for an entire series. And let's not forget he's not a person who plans things out, the fact he was able to make the power scaling somewhat relative is an amazing feat in itself. The idea that someone can't do a better job is ludicrous though. Clearly spending thousands of hours on the issue would put you in a position to do a better job than someone who spent a mere few.
2. Characters must be able to use their ki to brace themselves for oncoming ki attacks. Even a 1.25x KHH should, on paper, seriously hurt Piccolo if he takes it head-on there, but it doesn't. In this case, and in the Vegeta case, this is attributed to the characters' toughness, and we can indeed say that the characters are tougher than expected, but we can also assume that they're good at defending themselves with their own ki.
Amps are piss weak until the Androids Saga imo. That's the narrative behind them. A 2x amp is not piss weak.
3. You can probably assume that Goku improved the amp between 23TB and Raditz a little.
I meant what's the difference between regular Kamehameha and Super Kamehameha. If it's huge, you just completely changed the narrative of all events.
It's also important to remember that the Raditz fight was written immediately after the 23TB fight. Obviously there's stuff from the 23TB that Toriyama completely fucked up, like the humans' levels relative to Piccolo, but those levels weren't important when he wrote them in. His vision of what the Super KHH is capable of is probably misguided, but should be given more weight because it's used in a fight.
It simply doesn't work. If you do the numbers and walk through the scenarios I've presented, you will come to that conclusion as well. It's unrealistic for the author to be expected to have a perfectly rational ki amp progression throughout the series he was making up as he went along. We have far more resources and time to actually make something halfway feasible.