Sorry, but I think that isn't the case here. Rather psik raisha doesn't apply here because there's no actual psik raisha. When you cut a head it always kills it. When you cut skin it doesn't always remove tzaraas and hence the issue doesn't apply.
In each particular case, you need to examine if the action done for permitted reasons will always result in a violation of the issur.
1) In the case of the tzoraas on the orloh, the tzoraas IS on the orloh. In that case, there cutting off the orloh will always remove the tzoraas. So we need the Rashba's explanation to redefine the basic issur of removing tzoraas.
2) In the case of modeling clothing containing shatnez, the model IS wearing the clothing! There is no way to model the clothes without wearing them on your body. So the Ramban has to redefine the basic issur of wearing shatnez.
3) In the case of shearing off the black part of the hair of a Poroh Adumoh, You ARE shearing off the animal's hair. There is no way to remove the black part hair without shearing it off. So you have to fundamentally redefine the issur of shearing hekdesh animals along the lines of Rav Chaim.
4) Plucking the berries off a myrtle branch DOES make the branch fit for the mitzvah. There is no way to remove the berries without making it fit. So you need the Ramban to redefine tikkun moneh in a way that allows the plucking berries not to be considered making the branch fit for a mitzvah.
But I concede your point in two cases. Making a puncture wound in a korbon and pressing the cloth into a broken spigot. It is conceivable that those activities would not always automatically result in an issur.
I think you misunderstand. Cutting skin does not always result in removing tzaraas even if this case it does. In contrast cutting heads will always result in death. The obvious difference.
That's pretty good! It also works for making a puncture wound on a korbon which doesn't always form a permanent blemish. I think I heard this idea somewhere else...
You are absolutely right. Of course you can't be mechaven for the issur! You have to be mechaven for something else. I didn't realize what I was saying at the time, but now I deleted it.
Btw the purpose of cutting off the head of the chicken isn’t to give it to the kid, rather it’s to tease the kid with the head of the chicken.(At least according to the rambam).
Sorry, but I think that isn't the case here. Rather psik raisha doesn't apply here because there's no actual psik raisha. When you cut a head it always kills it. When you cut skin it doesn't always remove tzaraas and hence the issue doesn't apply.
I think I conceded too quickly.
In each particular case, you need to examine if the action done for permitted reasons will always result in a violation of the issur.
1) In the case of the tzoraas on the orloh, the tzoraas IS on the orloh. In that case, there cutting off the orloh will always remove the tzoraas. So we need the Rashba's explanation to redefine the basic issur of removing tzoraas.
2) In the case of modeling clothing containing shatnez, the model IS wearing the clothing! There is no way to model the clothes without wearing them on your body. So the Ramban has to redefine the basic issur of wearing shatnez.
3) In the case of shearing off the black part of the hair of a Poroh Adumoh, You ARE shearing off the animal's hair. There is no way to remove the black part hair without shearing it off. So you have to fundamentally redefine the issur of shearing hekdesh animals along the lines of Rav Chaim.
4) Plucking the berries off a myrtle branch DOES make the branch fit for the mitzvah. There is no way to remove the berries without making it fit. So you need the Ramban to redefine tikkun moneh in a way that allows the plucking berries not to be considered making the branch fit for a mitzvah.
But I concede your point in two cases. Making a puncture wound in a korbon and pressing the cloth into a broken spigot. It is conceivable that those activities would not always automatically result in an issur.
I think you misunderstand. Cutting skin does not always result in removing tzaraas even if this case it does. In contrast cutting heads will always result in death. The obvious difference.
That's pretty good! It also works for making a puncture wound on a korbon which doesn't always form a permanent blemish. I think I heard this idea somewhere else...
Edited...
You are absolutely right. Of course you can't be mechaven for the issur! You have to be mechaven for something else. I didn't realize what I was saying at the time, but now I deleted it.
Btw the purpose of cutting off the head of the chicken isn’t to give it to the kid, rather it’s to tease the kid with the head of the chicken.(At least according to the rambam).
How do you see this in the Rambam's words לצחק בו הקטן? Is it because he didn't write לקטן לצחק בו?
Yup, it’s much more of a joke that I like to make, but there might be some truth to it🤷♂️