Octopi DNA is different from human DNA. There is a gulf between the two that is inviolate. What that means is, if you have AB, you could concievably get BA, AA, or BB, but how in the world are you going to get ABC, if C doesn't exist somewhere in the genome already, even in junk DNA? But macroevolution is dependent on making a C where a C doesn't exist and moreover, can't exist.
Unless my knowledge of biology is way off, what you've said here isn't accurate. Octopi DNA is made from exactly the same things as human DNA, and any other DNA. The basic building blocks are Adenine, Cytosine, Guanine and Thymine (AGCT). Where is it that you think the fundamental difference is? What is the 'C' in your example?
In the same token, they share 96.5% similarity with a field mouse and 52% DNA with a banana! That doesn't mean that any one of us evolved from fruit.
Well I'm not a geneticist so I think i'll have to leave this to someone more qualified to explain, but from what I've read it's a lot more complicated than simply looking at similarities. For a start you can never say "organism A descended from organism B", but you can work out if organism A and organism B were likely to have shared a common ancestor, and maybe even what characteristics that common ancestor would have had.
Oh, I agree that the possible combiniations are inconceivably great, much more than the 'combination safe.' But if you have the number 100, there is a finite number of combinations available to you. Therefore, there really is a brick wall. But I also agree that a seemingly modicum of change can have irrepairable consequences. But that's another you need to consider. The vast preponderance of genetic mutations are either neutral or horrifically detrimental.
Well yes I do agree that there really is a 'brick wall', defined by the limit to the amount of genetic material that can fit into a cell. I have no idea what that limit is, but I imagine it is so large as to be meaningless. There's plenty of room to swing that cat!
As to detrimental mutations, I don't think there's any objective way to look at a mutation and say whether it is detrimental or beneficial. If a mutation makes no functional changes to a protein then it is neutral, that is objective. Detrimental and beneficial are highly dependent of the current environment of the organism though. What would be detrimental in one place and time could be beneficial in another place and time. It isn't a black/white situation, it's many shades of grey.
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