The host just delivers the Silverlight app in this case, so it should make no difference where it's hosted
First, I don't believe the "A" link would work in any host because it will look for A.XAML, not About.xaml.
Put this line
<urimapper:urimapping uri="/A" mappeduri="/Views/About.xaml" xmlns:urimapper="#unknown" />
above this line
<urimapper:urimapping uri="/{pageName}" mappeduri="/Views/{pageName}.xaml" xmlns:urimapper="#unknown" />
I tested it on both hosts - all links work fine, so if it's still not working for you then either the XAML for the pages you added aren't in the right place, the pages you added are not of type "System.Windows.Controls.Page", or you're typing the URL wrong.