1) As a user is scrolling, you can use PushState to update the URL to provide actual linkable pages. It does make re-clicking that link in the future odd though because that page's results are now at the top of the screen so scrolling up doesn't really make sense, but it at least provides the ability to deep link.
2) Sites like Pinterest solve this by opening an item's details page on click in an overlay above the infinite grid. That way when you close it (or click back in your browser) the modal just gets dismissed and you're right where you left off in the infinite scroll.