For showing JSON to your end user this marketplace component can help you: https://marketplace.mendix.com/link/component/105713
For the retrieve you can do it faster by ticking the First instead of all. Then you do not have to use the Head function.
For the places it is not working did you set a breakpoint and inspect what is getting returned? And do note that created date is not always set for an entity. Could it be that you changed that after you already created data? Because if this box was not ticked for an entity and you tick it later it will not be filled.
And you could use either the string you already retrieve from the webservice (so store as string first before doing the import mapping) and store that string somewhere to feed that to the JSON viewer widget.