I am sure that someone will come along with a much better answer eventually, but this is what I know from my teachers and my own reading.
1) We are already enlightened. Enlightenment is not so much something to attain, but rather we awaken to our own enlightened mind by discovering the nature of our mind. I don't know if this is unique to Shingon Buddhism, but so far it seems atypical to me.
2) shingon.org is a decent place to start for a basic overview. If you want to dive in deep, look into Kukai: Major Works.
3) There are a handful of temples on the west coast. So far the only teacher I am aware of in the east is in Vermont. I think there are a handful of licensed priests scattered about the midwest and the east coast, but no temples that I am aware of. If you can find one, you might be able to convince them to start a Daishi-ko if they haven't already.