I'm really glad that you found a way to get your photos back!
I'd really want to check with the other person who uploaded pictures and did their editing, as it sounds like it's possible that they maybe didn't understand how things would work and may have deleted your photos - possibly in an attempt to sort of clean up the account and make it easier to edit their own. I know Disney's older online Photopass site didn't allow you to delete the original photos - only ones that you'd edited as copies. At the time, it was annoying to me because I couldn't get rid of the obviously bad pictures, but in hindsight, it was a good protection!
If there's any chance the other group member deleted them, I'm sure it was because they didn't understand the ramifications, but I'd want to be sure that they do understand before anyone else uploads their pictures and possibly has the same problem.

I really wouldn't think they had any ill intent but I could see where someone didn't understand and since your photos were the first uploaded and would have showed up first, they may have deleted them to get them out of their way so theirs would show up first and be easier to see and edit.
The more I think about it, I really suspect that this is what happened, and I strongly suggest that other members of the group follow up on the issue within the group to be sure it doesn't happen again.
Probably a good idea to remind everyone that they should only edit or delete their own photos and not touch anyone else's at all. It seems obvious, but I could see that someone might not understand how it works.