Whatever criteria you are using to justify it's a good idea is your own subjective criteria. There is no good idea function cooked in reality which is objective across every observer.
But that's a fruitless line of thought in this context.
It doesn't negate the fact there are bad ideas. Heaven's Gate mass suicide was a bad idea (and its justification was patently wrong: no soul was uploaded into the mothership). Bringing feral tigers into your apartment is another bad idea. They are only "subjective" in the most theoretical, I'm-having-an-internet-argument sense; in practice they are objectively bad ideas.
Some ideas -- maybe most -- are neither good nor bad. But again, this doesn't negate that some ideas are objectively bad or good.
Hardcore moral relativism (or is it "ideas relativism"?) is such a boring position to hold. I suspect it's also a dishonest position, in that deep down very few people actually hold it.
From your POV and mine yes.
From their POV no. Clearly that's why they did it.
I agree what they did was probably a bad calculation and incorrect interpretation of the world given what I and you know about the world.
So from our POV it was a bad idea.
And from their POV it was a good idea and that's why they did it.
If they can reflect on it now they would likely call it a bad again.
Again subjective based on what they know about the world now.
No. You're confusing what someone mistakenly believes with the quality of the idea.
In this case, the POV doesn't change that this was a bad idea. Someone can think at the time something was a good idea, but this doesn't make it so, especially if it was demonstrably a bad idea. Few cases are clearcut, but this was.
The Heaven's Gate cultists thought their souls would get uploaded, that's why they killed themselves. Their souls weren't uploaded; therefore by their own logic committing suicide turned out to be a bad idea. They just aren't here anymore to reconsider, which demonstrates it was worse than a bad idea: it was a terminally bad idea.
Another example that POV doesn't necessarily change the quality of an idea: a toddler finding and handling their parent's loaded gun is objectively a bad idea (as is the parents leaving loaded guns within reach of toddlers, if they did it on purpose out of carelessness). The toddler may not know, from their POV, that this is dangerous; but this doesn't make it a good idea.
Some ideas can change with the POV, but you're mistaken in believing every idea does.