[O] Help with fixing an org-mode multicolumn implementation - LaTeX special characters

[mcg]

I found a very nice implementation of LaTeX multicolumn functionality in 
this thread:
https://lists.gnu.org/archive/html/emacs-orgmode/2013-02/msg00736.html
("using export filters to emulate multi-column table cells?")

#+begin_src emacs-lisp :exports none
(defun my-latex-multicolumn-filter (row backend info)
  (when (org-export-derived-backend-p backend 'latex)
    (while (string-match
"\\(<\\([0-9]+\\)col\\([lrc]\\)?>[[:blank:]]*\\([^&]+\\)\\)" row)
      (let ((columns (string-to-number (match-string 2 row)))
            (start (match-end 0))
            (contents (replace-regexp-in-string "[[:blank:]]*$" ""
(match-string 4 row)))
            (algn (or (match-string 3 row) "l")))
        (setq row (replace-match
                   (format "\\\\multicolumn{%d}{%s}{%s}" columns algn 
contents)
nil nil row 1))
        (while (and (> columns 1) (string-match "&" row start))
          (setq row (replace-match "" nil nil row))
          (decf columns))))
    row))
(add-to-list 'org-export-filter-table-row-functions
'my-latex-multicolumn-filter)
#+end_src

#+LATEX_HEADER: \usepackage{multirow}

You have to put the number of columns to span and a r / l / c option for 
column alignment (<2colc>) and the column contents afterwards. Nice!

#+CAPTION: Example table
| Characters not working        | <2colc> Characters working: ! " § . , 
* + ? % |                  |
|-------------------------------+-----------------------------------------------+------------------|
| ^ % _ { } $                   | The header above is 
merged                    | over two columns |
| /italics/ *bold* _underline_ 
|                                               |                  |
| any, \LaTeX special character 
|                                               |                  |
| or expression requiring \ 
|                                               |                  |

However, using any of the characters of the left column of my example 
table in the merged <2colc> column, I get:
"setq: Invalid use of `\' in replacement text"


Questions:
- How to escape the special characters in the left column properly to be 
able to use them? (tried everything I could think of)
or
- How to modify the code above so that it allows for such characters.

- If anyone likes the implementation and is more capable than I am, then 
the same for multirow would be really nice...

Comment: whoever read some of my questions recently will notice that I 
excessively use org-mode as a LaTeX frontend.  I am aware that this is 
not what it is supposed to be in the first place - but it works so 
perfect 99 % of the time, so I cannot keep from asking...