[kovidgoyal]

1) you'd have to ask a lawyer

2) Not out of the box, but it would be a relatively simple patch to the calibre content server code to have this kind of functionality. Basically you'd maintain a list of "checked out" books and not have an error message when the user tries to get it from the server. You'd also have to write an interface to "return" the book. But be aware that this system has no way of preventing someone who checks out a book from copying it.