Really, whoever.
So - either this group of drones were super geniuses (nothing showed or implied they were) or the collective at large already had transwarp (as it's established in Voy).
"Descent" does nothing to support the latter idea. So it's fallacy for you to use "Descent" to establish anything about the Collective's transwarp capabilities.
It's a separate matter, examined later on in case you missed it, that
other episodes establish the Collective capacity. But that in turn doesn't establish anything about "Descent".
The borg evolve - technologically - quite rapidly, as long a there's someone more advanced to assimilate.
And what is your evidence for that? The Borg technology seen in "Q Who?" and the Borg technology seen in "Endgame" don't appear to demonstrate noticeable differences - at least when you include the later episodes' retroactive establishing of many "classic" Borg features "Q Who?" was neutral about, such as nanoprobes, in ancient times already.
The only thing that could make it clearer that they were stronger than the federation would have been to be told directly they were stronger than the federation.
They had more advanced tech, were far more successful against far more borg than the federation, etc.
Your first point fails, as we see no technology more advanced than that the UFP possesses or, according to our heroes' expert estimate, could plausibly possess. Your second point fails as success against the Borg is merely an outcome, in no way associated with defensive capabilities in the episode; it cannot be used to counter the idea that the Borg cared just as little about Arturis' species as they do about Picard's, until they decide otherwise for reason X. And your third point... Well, you don't have any "etc".
There are species the borg really want to assimilate and species that don't have this "honor". The federation is among the latter.
As you yourself helped establish, there's nothing so binary about it. It's a spectrum of options, and sending of ships demonstrates interest at varying levels, whereas assimilation is but one of the things the Borg do for a living. Tickling the UFP with Cubes demonstrates interest - but it also demonstrates that the interest is not in rapid assimilation. And that's all we can say about that.
The borg said they'll assimilate their targets in First Contact (while heading straight for earth) and pretty much every time they were encountered afterwards.
And that's indeed the difference (possibly meaningful): the Borg merely identify ships as targets in the other encounters, and the assimilation of a species or a world seems to recede as an option, let alone a priority.
It doesn't prove anything, but it's a semantic point we can use for supporting varying ideas about what is going on. Or then let it drop.
Ok, but why would Picard receive the information necessary to defeat the Cube ? Was it by accident ?
If we're witnessing a "single timeline" drama, similar to Back to the Future, where only one timeline exists and is messed with (despite Doc Brown's rantings), then we must assume the Borg got what they wanted. Otherwise, they would have kept on meddling with their time machine. Thus, the loss of the Cube and later the Queen must have been a favorable or at least acceptable outcome, and in that case was unlikely to have been an accident as such.
If it's a multi-timeline thing, then perhaps the Borg failed here, in a series of accidents that gave Picard the advantage, but succeeded in some other timeline we didn't see. The heroes live happily ever after in this timeline, the Borg do so in the other one.
But "telepathy" doesn't strike me as the only possibility. We know UFP medical technology cannot remove even crude, macroscopic instruments such as the Wire from a brain; it's implausible that Crusher and her team could have fully cleaned up Picard in the aftermath of "Best of Both Worlds", despite their claims. Picard's blood might still be full of undetectable nanoinstrumentation, and it might only occasionally gather up into larger-scale, detectable machines such as subspace hotlines.
Timo Saloniemi