What files are created/modified when adding a user (useradd) in linux?

It depends on what NSS system you're using. If it's a standard unix login, typically /etc/passwd and /etc/shadow only. /etc/group is only used if creating a new group or adding the user to supplementary groups.

The files from /etc/skel are typically copied into the new user's home directory, which need not be /home/user

Things such as mailboxes and crontabs are generally created in an on-demand basis, so not when the user is created (the mailbox is normally created when the user receives their first item of email)

That sounds like about it... As far as I know, other resources (like mail spools and crontabs) are all created on demand. The home directory (copied from /etc/skel) and passwd/group/shadow files are all that should be changed on use creation.