For best results I recommend ePub with Adobe page template, which can be enabled via checkbox in the Layout compile option pane. This will add margins to the layout so that text doesn’t run all the way to the edge of the screen.
On the second question, try going into your Finder preferences, “Advanced” tab, and enabled “Show all filename extensions”. There should be an extension, but if not you can always type in the “.mobi” yourself in Finder. With that checkbox enabled, you should see the extension in Scrivener’s compile file selection window, as well as in all file dialogues across the operating system. I prefer it as it keeps things unambiguous.