Re: Amazon Tech level
Technological advancement is complex, but can essentially be boiled down to a couple basic requirements. You need resources (time, appropriate equipment/source materials, apples to fall on your head) and you need motivation (one invents the house because it's dangerous to sleep outside). Amazons have both. Why would the amazons have an airplane? Because they fight harpies. Sooner or later they'd start looking for a way to take the fight to the harpies' level.
Another aspect of tech development is, as you mention, contact. Exposure to new ideas. Amazons don't interact with other cultures, no. But they do have something that's just as good, that no human will ever be able to replicate: Immortality.
See, cross-cultural contact is the same kind of thing as interdisciplinary collaboration - it's a new perspective on an old piece of knowledge. This happens all the time in modern science - a chemical engineer and a mechanical engineer get to talking, and suddenly one guy's term paper becomes the other guy's million-dollar patent, because his background gives him a different take on the applications of what the first guy knew. In our world, you need at least two parties for this to happen. In the amazon world, it can happen to one - because once an amazon learns everything her culture knows about chemical engineering.... she's still got another couple thousand years to learn everything her culture knows about mechanical engineering. And she can combine all of it, not just what one person is able to explain to another. And then she can learn biology. And she probably started with theoretical math.
Amazons have all the time in the world. That's potent as hell.
Also, they're incredibly religious and their primary patron is the god of intelligence and innovation. Don't underestimate the power of religion in shaping a culture.