Is it possible to see all tasks (dated and undated) at the same time?
If you're using the web app, click the 'more' menu beneath the search box (above your Projects) and select 'View all Projects'. All projects will be displayed, arranged alphabetically.
Is there a way to set this as the default view? Paricularly in the Windows app?
In the web app, yes; in the Windows app, I haven't found a way.
In the web app, go to the 'gear icon' (settings), click on 'Settings', under the General Settings / Start Page, click the drop down menu and select Queries / Customer Queries. Then click 'Edit' and in the dialogue box enter 'view all'. Remember to save (!) and close the dialogue.
When you hit your start page, you should see all your projects.
As Randy stated, that is how you set your start page, when you do that through the web-app, it will be the same start page in the windows app.
Is it possible on the iOS app?
Unfortunately, it's not possible on the iOS app right now. We are in the process of creating a whole new iOS app that will address many of these issues and suggestions.