Install other applications in a WP7 application Programatically

Question

What the WP7 application needs to do?
- download some others applications and install them automatically on the device. The device will send first a request to a web server with some rules and will receive an answer containing the links to the applications that need to be downloaded and installed on the device.

As far as I read, the Marketplace is the only way to get apps on a Windows Phone 7 device, so let's suppose that the applications I want to install are already on the Marketplace and I can refer to each one by their links (according to this post).

1. Can I download programmatically an application from the MarketPlace (having the link to it) in another application and install it on the device? I do not want to open the app link in browser because this opens the Marketplace hub and points me to the application location.

2. Just for my curiosity: every time you want to install an application from Marketplace you need to sign in with a Windows Live ID?

Thanks & Regards, Paul