Hello,
The first question is not supported because you can easily make a copy of the original file, remove the rows you do not want and upload the file as desired. As for the second question, I believe the problem may be the item type you are basing your data load on. Are you using Work Item? Try others like RelatedWork entity field. This will allow you to base your upload on Resource, updating the 'Related Work' of the Resource. If the Resource does not exist, it will be created.
Please provide a more detailed example of your needs, more specifically what data are you uploading and how. Attach your file data (if you can) but if we need to investigate, you may need to open a ticket for further investigation.