ox-koma-letter.el: subtree vs buffer, precedence of properties, superscript transcoding

[Myles English <mylesenglish@gmail.com>]

Hello,

I have noticed some strange things about koma letter exports (current
git master 329683).

1) subtree scope export results differently than buffer scope export

2) a signature specified in the subtree property does not take
   precedence in subtree export

3) superscripts on the dates behave differently in the subject and
   date properties, i.e. 20\textsuperscript{th} & 20^{th}

The files included below constitute a Fairly Small Working Example (a
FSWE) that illustrate those things.  Using those files, I start emacs
like this:

emacs -Q -l config.el a.org   # or b.org

Thanks,
Myles


The Files
---------

1) a.org  Export like this: C-c C-e C-s k o   ________________________

* Letter
:PROPERTIES:
:EXPORT_LaTeX_CLASS: my-min-letter
:EXPORT_LCO: mylco
:EXPORT_TITLE:
:EXPORT_PLACE: Here
:EXPORT_SUBJECT: The thing that happened on 20\textsuperscript{th} June 2015
:EXPORT_DATE: June 20^{th} 2015
:EXPORT_TO_ADDRESS: Orgsters\\Internet
:EXPORT_OPENING: Dear Mr X,
:EXPORT_AUTHOR: Ronald Reagan
:EXPORT_CLOSING: Yours sincerely,
:EXPORT_SIGNATURE: This is the sig I want
:END:

The thing that happened was completely unacceptable.
SUBTREE EXPORT

2) b.org  Export like this: C-c C-e k o       ________________________

#+LaTeX_CLASS: my-min-letter
#+LCO: mylco
#+TITLE:
#+PLACE: Here
#+SUBJECT: The thing that happened on 20\textsuperscript{th} June 2015
#+DATE: June 20^{th} 2015
#+TO_ADDRESS: Orgsters\\Internet
#+OPENING: Dear Mr X,
#+AUTHOR: Ronald Reagan
#+CLOSING: Yours sincerely,
#+SIGNATURE: This is the sig I want

* Letter
The thing that happened was completely unacceptable.
WHOLE BUFFER EXPORT

3) config.el                                  ________________________

(add-to-list 'load-path
	     "~/.emacs.d/plugins/org-mode/lisp")
(add-to-list 'load-path
	     "~/.emacs.d/plugins/org-mode/contrib/lisp" t)

(require 'ox-latex)
(global-set-key (kbd "C-c C-e") 'org-export-dispatch)

(eval-after-load 'ox '(require 'ox-koma-letter))

(eval-after-load 'ox-koma-letter
  '(progn
     (add-to-list 'org-latex-classes
                  '("my-min-letter"
                    "\\documentclass[foldmarks=false]\{scrlttr2\}
     \\usepackage[UKenglish]{babel}
     \[DEFAULT-PACKAGES]
     \[PACKAGES]
     \[EXTRA]"))
     (setq org-koma-letter-default-class "my-min-letter")
     ))

4) mylco.lco                                  ________________________

\setkomavar{fromname}{Name in LCO file}
\setkomavar{fromaddress}{My house\\My street}
\setkomavar{signature}{\usekomavar{fromname}}