1) I think export will depend on the export type. Export to TXT or XML won't include the attachments. Export to HTML could optionally include the images.
Regarding your concerns about not being able to read .db file: this is a regular SQlite database file which can be read by any SQLite management application, such as SQLite Manager Firefox Addon (https://addons.mozilla.org/en-US/firefox/addon/5817/)
So at least you'll have some kind of access to your data.

2) Notes will have unique IDs which will be used as folder names inside of attachment folder, so no such collisions will be possible.

3) The attachment folder will be inside of Dropbox folder and DropBox will automatically take care of that.

Simple file references are already possible with upcoming 1.3.