How could it be possible to display all elements of a list (created by the FeatureMerger transformer) in a KML Description Balloon?

My first thought is using the ListConcatenator transformer to create one attribute per list attribute (comma separated for example) and display that attribute in the KML Balloon.

The balloon description is generally html.  You should be able to produce a fragment using the XMLTemplater and the fme:get-list-attribute function.

 

 

For example

<ul>
{for $x in fme:get-list-attribute("_list{}") return <li>{$x}</li>}
</ul>

 

You would then use the KMLPropertySetter and set the Description Balloon Content Type to HTML and the Content to the attribute generated by the XMLTemplater.  You would probably not want to Include the AttributeTable.