Can I use the editor to create arbitary key value pairs?
Is there another editor I can connect to update the profile?
Is there one that supports autocompletion with vocabulary prefixes/uris?
Yes, you can add pairs to a profile but be careful since, at the moment, if your profile becomes unreadable, you won’t be able to use it to authorize yourself and will be locked out of your account (contact your IDP if this happens).
As for editorrs, https://github.com/jeff-zucker/solid-ide provides the Ace/Cloud9 editor that does some syntax highlighting but is not much help with RDF. Keep your eyes on What's best triple editor on the Web that suits SoLiD development? which will provide other answers to the editor question.
I’ve copy pasted the profile/card source into a file which I keep locally and named profile.ttl. Then I used VSCode with the Stardog RDF Language Extension plugin active and found this helped me while editing the source code in turtle format. The plugin does highlighting and some tool-tips are provided. Once I was finished editing, I copy - pasted the source back into the Solid Pod source editor for my card.
See here for the plugin: https://marketplace.visualstudio.com/items?itemName=stardog-union.vscode-stardog-languages