My biggest concern is if a user closes their browser window while another peer is streaming some content. I'm assuming Peer CDN connects to someone else or uses Http's Range to fetch the rest of the file off the server?
Yes, that's exactly how the fallback will work. You can download content from 1 or more peers. If all the other peers disconnect, then it'll fallback to the origin HTTP server and fetch the remaining bits using RANGE requests. There are a whole bunch of reliability checks to ensure that files are downloaded correctly, completely and as fast as possible (re-downloading as few chunks as possible or not at all).
