I've spent some time looking through this VERY long thread as I too was interested in deleting photos from the photostream, however after looking into the purpose of photostream, it seems everyone here has missed the point of it…
For example, think of photostream as a replacement for a camera's memory card. When you're out, you snap as much as you like without paying attention to the quality of the photos you've taken. When you get home and plug your camera into your Mac, you pick and choose the photos you want to keep. Most of the time you delete the memory card after you've imported your photos because otherwise your memory card will run out of space.
Photostream doesn't run out of space however, it recycles space when you hit a 1000 photo or 30 day limit. The great thing about photostream is that your photo's are automatically downloaded and saved to your Mac to ensure you never loose them and if, for example, you want to keep a photo taken with your iPhone on to your iPad, save the photo to your camera roll to ensure it doesn't get deleted.
A lot of people are concerned about the security of photostream, but frankly why is no-one worried about the security of their contacts, calendars or their emails? They're all protected with the same username and password. If everybody sees photostream as a temporary memory card that you can pick and choose from, there will be no need to worry about deleting individual photos from it