You could try that, but as was stated; using -repair/-image is trying to fix the WHOLE .dat file. The -image tries to download any missing piece that you don't have, which wiki says could be 4+ GB. So depending on your internet connection it could take a long time to download. My understanding of the -repair command is that it just searches through the .dat file and looks for errors and tries to fix them. Again, this will take awhile to run through the whole file.
I don't think shutting down the downloader will affect anything. All it will do will stop and then resume the download from where you left off next time you run it. Think of it as if you were background downloading while playing, if you quit the game the next time you start back up it will continue where it left off.