You confuse imperfect definition with complete indefinition. If Bob says that he will meet Bill at the "foot of the mountain", that provides meaningful definition about Bob's future location. It is not complete: it's not sufficient to provide GPS coordinates, and it leads open a range of possibilites (which trail? for example). But it is sufficient to convey some meaning, and equally to convey some meaning about where Bob will definitely not be: the summit of the mountain, for example, or half way up the mountain, or Times Square, etc etc. Bob's statement therefore provides definition; it defines; it draws lines around what he does and doesn't mean. It is not complete: further definition is possible. But its incompleteness does not negate the definition that it does provide.
If I said for the sake of argument "I can't describe this likeness in full detail, and I think it's impossible to do so even in theory, but nonetheless I know that some sort of likeness between thought and reality is necessary for thought to be valid: thought and reality cannot be completely unlike", that would be meaningful as far as it goes. It would be sufficient to define my claim to a degree, and would be opposed to certain contradictory claims.
Now you show you have some idea of what "likeness" means in your response. In particular, your rhetorical flourishes about acorns, genetic signatures, death, and neutrinos, and how thoughts cannot be like their objects in the sense that they're from acorns, have genes, etc, show that some sort of definition has got across. You have some grasp of what likeness means. This is not surprising. Often we use words like "be", "thing", "fact" and "like" where the complete definitions may not be known, but the words and concepts nonetheless contain meaning. That doesn't mean we shouldn't seek more detail and greater clarity in our concepts -- we most certainly should. But it does mean that not being able to thoroughly define a word precludes its conveying meaning. This is especially true for common, everyday words such as "like". We use the word all the time; perhaps we can't define it, but we use it meaningfully nonetheless.
My claim above was very limited: that a true thought is, in some sense, like its object. It was a limited claim, using a definition of 'like' which, while incomplete, is enough to convey some meaning. Precisely how a thought and its object are alike is one thing, but that they are alike is something else. We could theoretically know that the latter must be true while knowing nothing whatsoever of the former. (To be clear, this isn't my position -- my position is basically Aristotelian realism -- but for the sake of argument, it could theoretically be true.) My claim that thought and object must be alike is not undermined by a hypothetical inability to explain how they are alike, assuming I present other reasoning that is sound. Just as someone without knowledge of the earth's rotation could tell you that it was day or night without telling you how it was day or night.
Much of the rest of your reply rests on the chain of causality theory which I address in my other post[0]. In short: a cause cannot explain truth in the mind, because any given cause faces the same question that the mind does about how it can be true, so unless you are going to attribute the cause's truth to its cause, and so regress into infinity, you need to find another explanation.
You claim that section 1 of [1] is moot. The first argument in support of this claim, that 'likeness' is too vague to mean anything, has failed. It does not get you off the hook to show that my argument -- that it's impossible for a thought to be both a) true and b) entirely unlike the reality it purports to be about -- is invalid.
More to follow on your other points when I get a minute.