In your case, it seems that you're attempting to use a singleton to retrieve information from one scene so that it can be accessed from some other scene. Rather than that, you probably want to store the information directly in the singleton, and then retrieve it in other scenes as necessary.
For example, in the autoload script
var playerHealth = 100
Then, in any other script:
Globals.playerHealth -= 10 # Subtract 10 from global player health
var myHealth = Globals.playerHealth # retrieve player health
So, rather than having the singleton retrieve information from other scripts dynamically, just directly store, retrieve, and update the information directly fromm the singleton.
Regarding your other question on string values. There's nothing special there. Put the data in the singleton, and then update, modify it directly from any other script.