Hi Mike,
On your first issue - the missing layers when attempting to select a data source: are the web layers a map or feature service? or are they something else (e.g., an OGC service)?
FYI, with the exception of the Map element, currently all of the other dashboard elements only support map and feature layers as input.
On your second issue - the "blank" appearance in the List element preview: this is expected behavior when you first open and configure the List element. Only the symbology that was set for the layer appears.
In the preview, the List element appears empty because nothing has been configured yet. If you click the List tab, this is where you can configure the content and appearance of individual items in the list.
Under the Line Item Text section, you can configure how items in the list will appear. You can access the web layer's attribute fields, by clicking the attribute field selector button:
As you configure your changes, the preview should update.
Hope this helps,