How to modify just added node after reading control properties?

After ReadProperties method is called, OnPropertyNodeAdded event can be used for fine adjustments of newly added node.

procedure TForm1.NextInspector61PropertyNodeAdded(Sender: TObject;
  Node: TNxInspectorNode6);
begin
  if Node.Caption = 'Variable' then
  begin
    Node.CaptionFont.Color := clTeal;
    Node.Pin;
  end;
  if Node.Caption = 'VariableLocation' then
  begin
    Node.Pin;
  end;
end;

Add another button inside Inlace Editor?

For this purpose use OnEditEnter event. This event occurs as soon edit starts. It provide handy parameters such as Node being edited and reference to the Inplace Editor itself.

procedure TForm1.NextInspector61EditEnter(Sender: TObject; Node: TNxInspectorNode6;
  InplaceEdit: INxInplaceEdit);
var
  Button: TNxExecuteButton6;
begin
  Button := TNxExecuteButton6.Create(InplaceEdit as TWinControl);
  Button.Parent := InplaceEdit as TWinControl;
  Button.Align := alRight;
end;

You are not limited to NextSuite controls, and you can place other visual controls inside editor.