Exactly, I figured out that the ACE permissions were overriding my POSIX permissions in this case after I had read the whitepapers. (excellent whitepapers btw, most thorough and concise I have found)
I don’t really want particular users to have blanket delete rights, as your solution suggests. I only want the owners of files and folders to have delete rights. e.g. user1 creates a file/folder and retains full read/write rights over that file/folder, including deleting. user2 can read and append the file/folder but cannot delete. I don’t want to have to manually manipulate permissions on each file in order to do this, but perhaps this is not possible.