Hello, Ron. Your post asks a reasonable question, and I’ll try to do it justice.
I agree that the “big tent” nature of ID creates problems for many people who encounter it. They often find it confusing or even self-contradictory. As you have pointed out, some of the lines of argument of IDers who are YEC clash with lines of argument coming from IDers who accept an old earth and common descent. I’ll try to clarify as well as I can.
I divide the ID movement into three main groups:
1 – the YEC-IDers.
2 – the OEC-IDers.
3 – the evolutionist IDers.
Examples: for 1, Paul Nelson; for 2, Steve Meyer; for 3, Michael Behe.
Now, the first two of these groups consist of creationists of various types, and therefore differ from the third group on a major issue – they deny the reality of descent with modification (beyond small changes within, say, families). And the first two groups differ from each other, in that the YECs deny much of modern geology and astronomy, whereas the OECs only challenge aspects of evolutionary biology.
So, you naturally ask, why are all these groups working together?
I see ID (understood as a theory rather than as a social movement) as a branch of design detection theory; it’s part of information science, in a way, or at least related to information science. Though all of these groups offer different histories of the universe, earth, and life, they all affirm certain methods to detecting design in nature. That is what unites them, across all religious differences (attitudes to the Genesis, etc.) and historical differences (how old the earth is, whether or not evolution occurred, etc.).
The two main contentions of ID have been:
1 – on the negative side, “Darwinian”, or more generally, largely chance-driven accounts, cannot explain the presence of the amount of complex specified information, etc. that we see in living things;
2 – on the positive side, nature exhibits features that we know, from our human experience, to be within the range of power of intelligent agents.
You will see these two points reiterated in all ID literature, with various wording, in various contexts.
It’s because Nelson, Meyer, Behe, etc. agree on these two broad points that they can all work together, despite major differences among them on other matters.
Thus, I see ID theory as neutral regarding the question “creation versus evolution”, but as taking a strong stand on the issue of “design versus chance.” This is why you can have, on the same Discovery website, endorsements of books which question the evidence for the descent of human beings from apes, but also, endorsements of books which accept that view. Both types of book are promoted because both argue that, whether or not descent with modification is real, the outcomes of the the creative process (whatever it was, a series of discrete miracles or a gradual transformation of previous forms) are such that we would not expect them to occur in the absence of intelligence.
Those are who are looking for a precise historical account of what happened in the past won’t find it in ID. Indeed, this is a criticism that many creationists make of ID, that it doesn’t offer a detailed narrative about the past, as creationism does. They say that ID is great on criticizing Darwinism etc., and useful, but not really a complete theory of origins, because it refrains from giving an alternate an anti-evolutionary historical account. So for them, ID becomes a useful tool, but is not really a strong origins position.
I think this weakness, if it is a weakness, is in the nature of the beast. By limiting itself to design detection, by conducting itself as a part of information science, ID exempts itself from offering specific details of exactly what happened, at particular points of the past. It’s concerned more about the formal than the material side of “design” in nature.
Does any of this help?