Scenario: I build, support, and host a website for a client. I want to let them manage their own email accounts, but I don't want them to be able to touch anything else within the cPanel.

I realize there's a feature to choose which options are available, but I will need to access many of those options. Is there some way to define multiple "levels" per a site's cPanel?

There is no way to do that in Cpanel.

Okay, thanks anyway.

You can give them access to email account through webmail - they can manage one email account only. But there is no option to manage multiple accounts for them