If you are looking for one report to one file, but the reports have different number of columns in different sections, then CSV can't handle it. Excel can by having multiple sheets, but you'll have to think about the problem a little. I have no idea whether you can write an XML file that Excel will load into multiple sheets. (Easiest way to find out, get Excel and try to dump a file with multiple sheets into XML format. Then study what it dumped.)
As for XML in the middle, that choice doesn't make much sense to me. Do you have any ideas why your boss is insisting on that?
Cheers,
Ben