Which characters are valid/invalid in a JSON key name?

No. Any valid string is a valid key. It can even have " as long as you escape it:

{"The \"meaning\" of life":42}

There is perhaps a chance you'll encounter difficulties loading such values into some languages, which try to associate keys with object field names. I don't know of any such cases, however.

The following characters must be escaped in JSON data to avoid any problems:

• " (double quote)
• \ (backslash)
• all control characters like \n, \t

JSON Parser can help you to deal with JSON.