Hi Prashant,
I am not quite sure of the setTagName issue, may be I will investigate more and get back to you.
For point#5, in order to update the chart with newly set chart AND query properties, make a call to chartobj.refersh(); This is same as chartobj.update(true);
As I have said before, when you write chartobj.update(false);, it only refreshes the chart with new display template properties, but does not reflect the new query template properties.
chartobj.update(true) will reflect and refresh the chart as per the dispslay template as well as query changes.
For you point #4, when you say it is not showing 'No Data' either, what is it displaying? Is it a blank screen?
-Rutika