In Emacs, how to inspect what has been modified in a buffer (before saving and overwriting the file)

Type C-u before either M-: or M-! to get the result inserted instead of sent to the echo area.

To get things directly into the kill ring, you need to dabble in Elisp. Something like this (untested):

;;; kill ring version of M-!
(defun shell-command-to-kill-ring (command)
    (interactive
      (list
        (read-shell-command "Shell command: " nil nil
                (let ((filename
                       (cond
                    (buffer-file-name)
                    ((eq major-mode 'dired-mode)
                     (dired-get-filename nil t)))))
                  (and filename (file-relative-name filename))))
        current-prefix-arg
        shell-command-default-error-buffer))
    (kill-new (shell-command-to-string command)))

;;; kill-ring version of M-:
(defun eval-expression-to-kill-ring ()
    (interactive)
    (call-interactively 'eval-expression)
    (kill-new (car values)))

You can put a function like follows into your .emacs and bind it to any key you like.

(defun insert-file-name ()
  "Insert the full path file name into the current buffer."
  (interactive)
  (insert (buffer-file-name (window-buffer (minibuffer-selected-window)))))

Also see this question, it seems to be similar.