The Borg ALWAYS had representatives, in nearly all their stories.
Q Who? Had Q and Guinan serve as their "reps". Explaining the Borg to the Feds.
BOBW had Locutus.
I, Borg and Descent had Hugh and Lore.
They are, they were there to explain the Borg to the Feds because the writers themselves knew that a truly faceless foe just isn't workable without someone there to "speak" for them.
You did not at first write that Guinan and Q served the literary convention of giving a mouthpiece to an otherwide anonymous enemy. You equated their roles, putting Guinan and Q into the same political position as Locutus. In a weak sense, Guinan and Q represent the Borg in that they explain what they can about the Borg. Locutus, on the other hand, is actually in the service of the Borg, their complete and literal representative. I guess that if I were teaching about about terrorists or guerrillas, it might be said--figuratively--that I represent what they stand for to the best of my ability. However, if someone said (borrowing your words), "Al Qaeda ALWAYS had representatives, in nearly all their stories," equating me with some radicalized figure, making cut-rate videos, spouting quasi-religious rhetoric while footage of training exercise played in the background, I would have every right to be furious. To call me such a representative would be a gross misrepresentation of the facts. Moreover, if Guinan was such a mouthpiece to the Borg that she could say with such certainty what the future intentions of the Borg were ("Guinan said that one day dialog with the Borg would be possible.
"), Picard should have thrown he in the brig as he investigated her as a spy: such direct knowledge of the Borg's intentions should have been suspect. Of course, Guinan doesn't know: she's speculating, though without much confidence. She isn't representing the Borg, not in the way you suggest.
As per whether the Federation should have been fearful of the Borg, the technological or metaphysical superiority of the race is not an indication of the threat that they might pose. Other than jockeying for geo-strategic advantage, the great powers--USA, Russia, China, UK and France--don't consider each other immediate threats simply because of their strength. Indeed, UK should not be threatened by USA in any way; whether the UK feels threatened by China should not be affected by the fact that USA has a military advantage over China. Indeed, over the last two decades, those powers have been threatened by lesser powers that might use force to create instability IN SPITE OF THEIR RELATIVE WEAKNESS. Some of those lesser powers aren't even nation-states; sometimes they are, at best, loosely organized. So I feel comfortable saying that the fact that their might be superior powers (and all your examples are debatable), that the Federation should not solely base their threat assessment of Borg solely on how they rank among the galaxy's power, but also based on what they intend to do and how they conduct themselves.