You can rewrite and remove symlinks and then also lose "information".
Perhaps I just didn't get what you meant with "security".
I meant that copied files are writable, while links to files inside a squashfs mount are not.
Also you can use squashfs tools to list complete package contents (including their paths relative to root) or download the same listing as a file from the repo if you want to know where an extension puts it's files.
This is true; however, there are performance problems with your approaches (I tried the squashfs tools way when working on my untcz script).
Moreover, your approaches are less robust: suppose you, for some reason, have no longer access to the original tcz file...
In such cases, if you used copy mode, you have no way of knowing what files belong to what extension (in this sense I talked about losing information), while if you mounted the extension you still can.