mIRC scripting is single-threaded*. Each command waits until the previous one has finished before it runs, no matter how long that is. There's no explicit return value, generally speaking if a built-in command fails it will generate an error which will either trigger the error handling routine if you've put one in (see Error handling in the help file) or else it'll simply halt execution of the script.
* - With the exception of $comcall() and $dllcall(), which chances are you're not using.
