Back to the original topic, neither can the SETI test for signs of extra-terrestrial intelligence. It's looking for signals that are likely artificial. IC is looking for systems that are likely created. But I should also point out that science accepts all sorts of ideas based on their probability of truth rather than certainty. That's why there are so many more theories than laws in science. In other words, from the other direction, science can't exclude a supernatural explanation or some other non-evolutional explanation for life on Earth, either. I think the level of certainty your demanding simply doesn't exist very often, even in real Science®.
If you've ever debated philosophy with a postmodernist, you'd realize that about the only thing we can really prove is "cognito ergo sum" -- "I think therefore I am". Almost everything else is conjecture because you can't exclude the possibility that everything you experience is artificial, a point played to great effect in movies like The Matrix.
No, I didn't say that. I only said that it is impossible to verify a designed process via your definition of IC.
It's only impossible if you rule out the ability to identify and understand the mutation paths necessary to get between two points. Given not only our growing understanding of genetics but our ability to reconstruct DNA from very old remains, it's no unthinkable that we will someday understand the starting point, the possible intermediate points, and the end point of a sequence of mutations and determine whether there is or there isn't a natural explanation of how to get between those two points because of the complexity of the changes. Essentially, you are making an appeal to ignorance here (which Carl Sagan's Baloney Detection Kid defines as "absence of evidence is not evidence of absence") -- we can't ever know if ID is true, thus we must assume it is false.
We are also blind to most of the em-spectrum. Radiation of heat is also part of the em-spectrum. If their shamans allow them to study physics they will discover em-waves.
We are blind to most, but not all. That's important. I again reference science fiction author Larry Niven's Kdatlyno, a race that uses sonar and touch but has no sight. They look up at their night sky to see only the end of their world because no stars or moons shine down on their eyes. Imagine doing electro-magnetic physics while totally blind.
As for "shamans", there are plenty of those in the sciences, too. Don't think that there isn't an orthodoxy and dogma in science. Scientists are human, after all.
They will keep on searching. Why? The possibility for live is not zero in the universe. If something happened once, it could happened twice, three times ... .
The possibility of God is not zero, either. The possibility that we are all brains in a vat, living life Matrix-style, is not zero. All you are really saying is that having one example of intelligent life crosses your own personal level of probability such that a search for intelligent life on other planets seems prudent to you. Against your single example, there are plenty of things pushing the odds in the other direction, including the Fermi Paradox and the large number of factors that could make a planet unsuitable for the development of life (remember that just because current life forms can exist in some very hostile environments does not mean that life could have started out in those same hostile environments -- the environment needed for life to first form could be so specialized that Earth is unique).
Going back to the original topic, that's why I said it's all about a gut level assessment of the odds. You believe that the odds were good that life evolved naturally on Earth and, thus, could have done so on other planets, even if you can't prove it beyond any doubt. Other people look at the same things you are looking at and assess the odds differently. You aren't being unreasonable, but neither are they. So long as it remains a guess, educated or not, one is not necessarily superior to the other.
I also want to know that if they failed to find an obvious EM signal, what other things might SETI search for?
That is the point where scientists draw the line between science and faith. Observations have to be reproducible no matter the observer is religious or not.
And that's fine. What ID is looking for is observable evidence of God via IC. Like I've said, that may be a fool's errand, but they are attempting to do exactly what you claim they should be doing to do Science®.
As I told you before, you don't need to know purpose or meaning of any em-signal. It's the physical structure of the signal itself.
And as I've told you, finding an EM signal with no known purpose or meaning only proves that you've found an EM signal that may not be natural. Applying your own test of validity, such a signal, without a known purpose, meaning, or method of creation, could not exclude that idea that it may have been formed naturally through some yet-unknown natural process. Thus, it would prove nothing. All it might do is increase the odds in favor of ET intelligence. Prove it? No.
ID and IC are Trademarks of the Discovery Institute (DI).