We're also adding a CLI utility to enter a website container by domain, which should be quite a big time saver. Until then, looking in /etc/passwd is a good solution. You should always ssh or su to the website user to work on websites, never do it as root. If you connect with SSH, you will automatically be placed in the correct directory.
The website paths won't change, they are deliberately based on the websiteId as mapped domains will change but the websiteId is always static, even after a server migration.