Why would God create a “very good” world only to allow it to fall so far that he had to destroy it (Gen 6:7)? Does not the talk of God’s “repentance” imply he had no knowledge or control over the state of the world?

First, let us get clear on this: to say “it repented the Lord” (i.e., “the Lord repented”) does not mean either that God admitted to himself that he had done something wrong or had made a mistake, or even that he had changed his mind on some point of principle. What had changed was man’s moral merits, and God’s attitude toward man reflected this moral fact. This still left a problem, which is really a variant on the problem of evil, unsolved. There is little in the Bible, as far as I can tell, that clearly reveals God’s purposes in permitting evil, although it is absolutely clear that he does permit it and also that he even achieves important purposes by permitting it. The book of Job clearly implies we must not expect to learn God’s purposes. But a theory that is at least consistent with the Bible makes it at least plausible: God did the least he could to help rebellious man until repentant man could be redeemed and purified by the perfect sacrifice. God kept a righteous remnant alive so this was possible—and thus he did not repent of his original purpose in creating man, namely, to create intelligent beings, acceptable to the Lord, who would freely glorify and take delight in the Lord. There is much more to the theory than that, but it is enough to suggest the answer to this question. By the way, the word translated “repent,” נָחַם or nacham, is the same as that used by Lamech in his prophesy saying Noah will “repent (nacham) us”; it is as if God were saying, “You were right, Lamech: Noah would bring “comfort (nacham)” in the sense of “repentance (also nacham),” although not in the way you meant. My repentance would put an end to your work and toil—with death.”