Once the incomplete files start downloading, they will resume at previous download %, but until then will show zero %. This was true for LW 4. Not sure about LW 5. Deleting the download.dat & bak files? Who suggested you do that? I'd like to see a quote. They are the files that maintain your download preferences & download history.

In LW 4 all you'd need to do is go to Library window, select incomplete folder, select all files holding down SHIFT key, press Resume button. This would force all incomplete files back into Download window.

LW 5 does not have a resume option. And canceling files in LW 5's download window will delete the incomplete files.