Is there a way to change the email address used for my Emby Connect account? I'm not too bothered if the username either stays the same or changes, but the email address is being disposed of shortly

No we don't. There is no limit to the number of people or time limit. However the types of clients you can use for free is more limited without having Premiere. Without a Premiere License you won't have some of the advanced Server features nor access to clients like Theater which are Premiere clients only. This link to our Premiere Feature Matrix might be useful to you: https://support.emby.media/a/solutions/articles/44001173099