>
[snip]


   > ;;;; Show the text/plain part before the text/html part in multpart/alternative

   > (Require 'mm-decode)

   > (Setq gnus-inhibit-mime-unbuttonizing t
   >       mm-discouraged-alternatives '("text/html"))

Thanks meanwhile I found out that the essential setting is 

(Setq gnus-mime-display-multipart-alternative-as-mixed nil)
(setq gnus-mime-display-multipart-related-as-mixed nil) 
(setq gnus-mime-display-multipart-as-mixed nil) 

Rationale for t values:
;; Comments: LaTeX: if you want to display automatically the png
;; pictures generated by `org-preview-latex-fragment' you should set
;; these three multipart variables to t. However this is very annoying
;; if you receive html mail, its text gets doubled. So it is better to
;; set them to `nil' and use the function `article-treat-org-preview'