seems reasonable that DE could harm the intestines but many people swear by it. i guess the damage might not show up for years. i googled around for warnings vs taking DE several months ago because of similar concerns but could find only a few webpages.
most of the rest of your post is just speculation. what kind of parasites would flee from DE? are protozoans smart enough? do they have ability to get into the blood and if so why wouldn't they do so before encountering DE?