Why are hook_form_alter updates being reverted?
The function seven_form_article_node_form_alterwas correctly named but not placed in the correct file in the correct folder.

The function seven_form_article_node_form_alter has to go in template.php in a (sub)theme of the administration theme seven - of course - and not in the  template.php of my "front-end" theme.    

The fact that I had a function in there with a prefix other than my from-end theme's should have signalled my mistake.

What mislead me was a quirk of Drupal cache clear. It happens that  clearing the Drupal cache actually (and incorrectly) caused my form alterations to be displayed as my function must have been the last one in the chain to have taken effect. It was only after a subsequent page build / refresh that the form alter's effects (correctly) disappeared.  So this is a quirk of Drupal cache clear that needs to be taken account of in this edge case.

Consequently I have created an administration sub theme into whose template.php file the function now sits (Does it count as one line of code or two? ;-). All is now working as it should.