Regarding the questions:
1) Yes, the filter component can be replaced with a custom one:
2) Currently, only the text of the items can be changed using the label prop:
3) This can only be achieved by programmatically subscribing to the click event of the button. Still, this will add the new expression in the tree. Please share more details about this use case, so we can see if we can offer something or log a feature request.
4) This is determined by the order of the items in the fields collection. If a specific field has to be shown when the add description button is clicked, then set this field to be the first one in the fields collection.
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and vote to affect the priority of the items