I will go straight to your questions:
1. What I can suggest you for this scenario is to check the following example from our online XAML SDK Repository:
The example demonstrates how to implement a minimum delay before the filtering is triggered. However you can use the delay in order to modify the ItemsSource as well. During the delay a busy indicator is shown below the AutoCompleteBox indicating the loading of the data.
2. About the button you can easily add it in the ControlTemplate of the AutoCompleteBox. You will need to extract the template and look for a Grid named PopupRoot
it contains a ListBox with the filtered items. You can add the button below the ListBox, bind it to a command in the ViewModel and toggle its Visibility when needed.
Hope this helps.
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