Is it possible to add users via the Splunk REST API in a SH cluster? I thought that I had tested this but now when I try to add a user it just hangs. Any help is much appreciated!
I'd really very strongly recommend using LDAP or similar things for authentication, maintaining users manually is a huge pain in any org large enough to require SHC.
That being said, anything you can do through the UI you can do through REST, see the other question you had: https://answers.splunk.com/answers/561658/modify-splunk-users-role-via-the-rest-api.html