I made a copy of the "Mail (original)" file, renamed it, & ran the update again.
No, doing that changed the Permissions on Mail.app. When you move a file or folder owned by the system using the Finder, the Permissions get changed. Get BatChmod to do this.
Drag Mail.app into the upper field next to File and set it up like this. Then hit Apply. Do not check "Apply to enclosed..." Or any other boxes in the screenshot, except Change Ownership and Privileges.
Or just do a Permissions repair from Disk Utility and see if that fixes it. Inspect the messages you get from Permissions repair and be certain there's one about Permissions for Mail.app being repaired. Disregard all the other messages you get. When finished, drag Mail into BatChmod and see if they match the screenshot below. If they do, just quit BatChmod.
You should trash the smaller Mail, the one with the generic icon (don't empty the Trash just yet.) Leave only the one with the proper icon in Applications.
There is now no enclosing folder, so don't worry about that.
See if Mail will open now. If not, then run the update once again.
Only the original poster can award points. Don't worry about it. It was a pleasure helping you and reward enough. Besides, I learned something too and that's very valuable to me. A grandmother eh, no idea!