In order to save server space, I want to only allow one copy of an image during upload. I've already hidden the multiple embed codes, where would I modify the extra copies?
No, I won't ever consider to get rid of the multi-upload stuff or even make it configurable. What's the point anyway? The uploader detects duplicates, php detects duplicates so you aren't getting duplicates, just images that looks exactly the same but the files aren't exactly the same.
I will say no. Medium image is crucial for fast listings and thumbs are also crucial for BBCode stuff. Thumbs could be configurable, but I really doubt in disable medium sized image.