I'll try and answer your questions as I currently understand them.
1: You can export only certain rows on the grid by first filtering the grid based on the criteria you wish to show in the export. For more information on filtering please refer to this help topic
. Once you have filtered and exported, you can clear the filters if you wish. In order to use this you need to set:
exporter.HiddenRowOption = HiddenOption.DoNotExport;
2: You can export only certain columns by hiding the columns before export, and showing them again after, and in your export options choose.
exporter.HiddenColumnOption = HiddenOption.DoNotExport;
3: you can format the output as it is being rendered by using the ExcelCellFormatting
Please refer to this help topic for more information
I hope that helps but please let me know if you have any questions