When you save the spreadsheet and the file exists, you hare the option to either overwrite the file, append data to the file, or skip the export. Assuming you select the middle, you have the option to overwrite existing worksheets or append to the end of the worksheet. I think the latter is what you are looking for.
Yes, there are several options you have to uniquely name exported spreadsheets. There are several macros such as a date, first value in a column, or even the filter / summary used. These macros can be used for file names or even worksheet names.
There is a lot of flexibility and room for your immagination to see what you can do.
Yes, but it becomes more complicated. You cant do the export directly to the spreadsheet you want. You'll need to do something like export to another sheet and then have a macro that copies the data to your presentation report with the logo. The macro can be called through a batch script that is called from the post export script in Data Pump.