I found a way to fix the problem, but I do not yet understand why things are done the way they seem to be done ...
I understand that when you click on on of the tags displayed for a page, doku.php is called with the name of the tag and the action request "do= showtag". This request is then somehow (sorry, I am yet not fluent in event handling) passed on to the function _handle_tpl_act() of action.php of the tag-plugin.
_handle_tpl_act() retrieves (with the function call getTopic($ns, '', $tag) ?) the pages with the desired tag and creates an array "$pages" with the info which needs to be displayed. It then loops through each page (line 104 of action.php):
foreach ($pages as $page) {
$pagelist->addPage($page);
}
The function addPage() belongs to the pagelist-plugin and is defined in its file helper.php.
addPage() uses different functions _fooCell() to preprare the output of the page list. The function responsible for preparation of the pagenames is _pageCell(). pageCell() has a section which handles "image and text titles". It is also here where it trys to decide whether to display the page name or the first headline (as defined by the config parameter "showfirsthl"). The trouble is: all this intelligence only comes into play if no title is provided in the beginning (the title can be provided in as the element 'title' of $page). The tag-plugin, however, appears to pass the first heading as the title to the pagelist plugin, which then displays this first heading dutifully.
So if you modify the above mentioned loop of _handle_tpl_act() in action.php of the tag-plugin to
foreach ($pages as $page) {
$page['title']=''; //necessary to let pagelist-plugin decide on title display
$pagelist->addPage($page);
}
the list is displayed as expected and you can switch between page name and first headline using the config parameter "showfirsthl" of the pagelist plugin.