To answer your second question, very likely. Microsoft would have zero luck populating their own Android store from scratch. It's a win-win for both companies as they're playing second fiddle to the Google Play Store and maybe even the Samsung store. Attracting developers to submit apps requires a large market.
What would be the reason to use android apps on windows besides them not being available on windows? What feature could an android app running on windows do that a windows application couldn't do?