Ah, now this makes things much clearer.
For one, you can use filter expressions in default tiddlers.
So, what you really need is a filter expression that gives you the desired list.
I would not use any shadow tiddlers for news, unless you just have
one News tiddler that lists them without ever showing their titles
or navigating to them.
I take it, you only want to show news no older than x days:
You could simply have a News tiddler that you put in your default tiddlers,
and then use that to list all your news according to that filter,
alongside a template that also shows a specified published date,
which is not necessarily either of modified or created.
Best wishes,
Tobias.