It is possible, you put the Text element inside the the header() method of Panel:

                Panel::create()
                ->header(Text::create("guide")->text(function(){
                    $selectedProductLine = $this->dashboard()->widget("ProductByLine")->selectedProductLine();
                    if($selectedProductLine!==null) {
                        return Html::p(["List of ",Html::b($selectedProductLine)," products"]);
                    }
                    return null;
                })->asHtml(true))

For your second questions, D3 has not fully developed for Dashboard like GoogleChart so the client events has not been available in form of "action" like google chart but you can do it like following (very simple).

D3 chart has the itemSelect event, here is the doc. So what we do that we will register the itemSelect event and then call the widgetAction() of Dashboard Framework, the widgetAction will help to call the actionItemSelect at server-side.

class MyD3Donut extends \koolreport\dashboard\widgets\d3\DonutChart
{
    protected function onCreated()
    {
        $this->settings([
            "clientEvents"=>[
                "itemSelect"=>"function(params){
                    widgetAction('MyD3Donut','itemSelect',{
                        selectedValue:params['selectedValue']
                    });
                }"
            ]
        ])
    }

    protected function actionItemSelect($request,$response)
    {
        $selectedValue = $request->params("selectedValue");
    }
}

Hope that helps.