Hi Praveen
Glad to hear that there is progress 
1) I've been thinking about this for a while, and I also haven't figured out how to suppress the main payload. If it is an XML payload, it is possible to add an XPath condition when determining receiver - but I think the mail payload is non-XML content.
2) Can you check if the filename is available in each of the child messages under it's content type? If it is, the content type would look something like below:-
Content Type: text/plain; charset=UTF-8; name=file1.csv
If it is available, then add the parameter storeFileName = Y in the AttachmentSplitterBean of the iFlow 1. This would store the filenames of the attachments in the default attribute FileName in namespace http://sap.com/xi/XI/System/File.
Once you have the filename in the child messages, add DynamicAttributeChangeBean (refer to scenario 2 on mode = change) on iFlow 2. You just need the following three parameters:-
mode = change
oldValue = .csv
newValue = .xml
3) Not sure what you trying to achieve with PayloadSwapBean. What do you mean by "multiple receivers for each attachment"? Maybe you can elaborate with an example?
Rgds
Eng Swee