1: the attributes in the infopage class are not the same as in the article class. Info page doesn't have an intro attribute for example, which means nothing will show up when you duplicate the article override for infopage.
2: I wanted that too and finally found a solution. When a user attempts to browse a folder the latest article in it is shown instead. How I accomplished this? I made an override for node/view/full.tpl matching the folder class, inside the template there is a fetch function with limit 1 that shows the latest article (using node_view_gui view=full) Under there you could print a list with the other articles using another fetch function. Read more about fetching here: http://www.ez.no/developer/ez_publish_3/documentation/development/libraries/ez_template/operators/data_fetch
1 - Your answer for my question 2, lightened me about the solution for the question 1. The problem is not in the line_infopage.tpl but in the folder.tpl (this is a override full.tpl for the class 1 - folder listing). Edited folder.tpl and find:
class_filter_array, array( 1, 10 )
10 is the class id for infopage. I change to:
class_filter_array, array( 1, 11 ) and the problem is solved.
2 - Ok, you find the solution! I have the way to fetch limit 1. Trying to show the article using the article.tpl ...