You don't.  Do the loop, but put in a filter condition right after the loop starts that prevents further execution once you've exceeded the condition you're after.

Why not just use the API to do a /count operation?  Count all proofs with a non-complete status.  When 0, proceed to process the task from there.

This is likely your main issue.  Would focus on setting a proper URL as per their API spec.

I don't have an AEM Assets connection, so I can't screenshot anything.  However, typically in the Custom API modules, there's a URL entry that points to a specific API endpoint (defined via mechanisms such as https://developer.adobe.com/experience-cloud/experience-manager-apis/api/stable/sites/).  T...

That's a very large string for content.  Typically, you'd set Content-Type to application/json and POST the body as the JSON itself.  What API are you trying to hit?

https://experienceleague.adobe.com/en/docs/workfront/using/adobe-workfront-fusion/functions-in-fusion/tokens-for-date-and-time-formatting 

You can totally do this.  Just setup one connection per Workfront instance.

It's best to open a support ticket when you think something is down, or check status.adobe.com.  The forums aren't actively monitored by Adobe Support.

That should work.  emptystring is the same as "" in most programming languages -- a blank string of 0 length.  It is different than null and ignore, though has a "falsy" aspect to it.  (Fusion's backend is JavaScript, so you can imagine there's some overlap there.)