-snipped for brevity-
To your first counterpoint; I can see where you're coming from, but the validity of this point hinges on whether the cube was possessed of enough sentience and intelligence to actually adapt in the first place, which I still contend runs contra to what we've seen from the Borg previously.
To your second; no, it has not been explicitly stated, as is the case with most Treknology, we must make logical inferences from the information that is available and what we can see on screen. Whether the original Borg concept was backwards or not isn't really at issue, they are an established fictional race with established features, and none of those established features support the concept of sentient cubes.
To your third; I am aware of what trauma means, my emo comment was meant as a joke; I assure you I do not regularly use words like "cubie-woobie"
Regardless, I still cannot see why this is a unique situation. There have been numerous instances shown where massively damaged cubes have been cut off from the Collective with most or all of their drone populations killed, particularly during the war with 8472, yet those cubes were never shown as anything other than inert technology. Wreckage.
I just keep coming back to the same thing; there has never been any prior indication that cubes are sentient, or that they need to be so. The Collective consciousness is the Borg, without that, nanites become dormant, drones revert to individuality or die, and without those to maintain it - technology becomes inert. For me personally, it's simply too much of a stretch to assert out of nowhere that, well, actually, cubes are sentient see? And they're made up of nanites, see? Oh yes, and they can also puppet drones, and make "pseudo-queens", and suck people into walls, and and and etc etc.
Agree to disagree I suppose.