I thought so, that's why I've added piece size. The less it is, the more pieces you have to upload, hence more ACKs & CRCs with torrent's data (not to 4%, I think, but nevertheless).
Well, maybe you have something checking for updates in background (e.g.Windows Update).
I don't think so, though I never tried. You may try on some small-sized task, if it's interesting for you.
THAT would certainly erase your U/D ratio & all stats for your task; about saving .torrent - you can use the one from the site (if you haven't modified it since opening in BitComet - e.g.adding/removing trackers) - either directly from there or from your HDD/wherever-you-saved-this-before-opening-for-a-first-time. Making .torrent out of task was required here to save any possible changes plus you don't have to look for such .torrent in, say, C:\Program Files\BitComet\Torrents or wherever to add it back later.