SecureCmdOptionParse only seems to return action,unit for dynamic macros.

For example:

/cast [] Something returns "Something", nil

However:

/cast Something returns "/cast Something", nil

This doesn't really make sense to me. Should I use something else to determine the action from non-dynamic macros?

This is not a bug, you're assumed to have pulled the options out of the command line first:
Usage: SecureCmdOptionParse("options")

This allows you more flexibility in the parsing process.