Of course VOY made it worse by giving us a glimpse of the hundreds of cubes the Borg appeared to have at their disposal. So why send one to attack Earth?
Because it's a very, very big galaxy and Earth is just one very remote part of it as far as the Borg are concerned. Given how vast their territory is, they must be at war with thousands of different civilizations at any given time as they strive to expand their territory. Our attention as viewers is focused exclusively on the Federation and its ships, but the Borg's attention must be primarily directed toward those thousands of other civilizations that we know nothing about because their battles with the Borg are taking place in what, to us, are remote and unknown parts of the galaxy. So the Federation was very, very far from being the Borg's highest priority. It was interesting to them, interesting enough that they wanted to assimilate it, but not so important that they would allocate more than a single cube at a time to the effort, not when their cubes were needed in so many other battles far closer to home.
Of course, Voyager
destroying the Borg's transwarp hub and Unicomplex would've changed that, raising the Federation from a source of curiosity and a minor nuisance to the level of a major threat. And in the Destiny
trilogy in the novels, we see what happens when the Borg really do make the Federation their priority.
Meh, I assume that any Borg ship has the ability to create the queen as needed.
As I see it, the Queen isn't really a person, just sort of a coordinating nexus for the collective consciousness. The physical body is just a substrate for what's basically a software entity. So any individual Queen body is expendable. If one is destroyed, a new one has the Queen software downloaded into it.