1. What is the solution for exporting headers of a datatable when the table header has commandLink associated with it (above mentioned problem)?

When this one is fixed, it should work.

2. I am not sure whether returning the listRow in the getWrappedData() is the right implementation or not. But it is working. Could you please show me a good example of extended data model which works with export to excel?

Extended data models (or RF in general) is not my area but I'd say If it works, it ain't broken

3. If i want to export 50,000 records instead of 10 records (no. of records on the web page), is it a good idea to call getAll() in getWrappedData()?

Again, not my area but the idea is give an iterator that will return the rows you want when iterated over in the normal way.