Deleting all copies of a photo If I delete a photo from both Camera Roll and Photo Stream, is there still another copy on my Mac or elsewhere that Photo Stream placed there?
It depends on the order of sequence.

When a photo enters Photo Stream, all devices that are on and running and set to download a copy can get the photo. In the case of your Mac, iPhoto can be set to automatically import the photo, so yes - it could have a copy stored in the library even though you have deleted it from the stream.

Deleting the photo from the camera roll doesn't have any influence on the stream or other devices other than perhaps if you were to connect the device over USB to a computer and import later, but I'm guessing that's not your concern.

In practice, there is some delay so if you took a photo on the phone and then deleted it almost as soon as you took it from Photo Stream, the delete likely would prevent another device from seeing the photo, but if your network is fast - the photo could be elsewhere.