But as with all things, you don't need anything like 100% coverage to get an app to work. Just handle the obvious stuff, and patch as needed until your target works. Then repeat. This is how WINE worked in the early days: you don't start with a copy of the win32 API, you start with a program you want to run and see where it fails.
Most apps, for obvious reasons, use only a tiny fraction of the platform. And those bits they do use aren't always critical. Facebook, for example, wants location information but would be happy if you just lied to it and gave it a hard coded value.
And -- as with WINE! -- games probably use much fewer of the overall APIs than a "desktop" application (like Facebook) would. There's no need for any of the complicated UI widgets, for instance.
Emulation is absolutely legal. There is a long history of emulators of all sorts in the market. And of course there was an important decision just last week that held without ambiguity that the mere interface to a program is not covered by copyright.
I'm actually curious about this as well, is this actually legal? If it is similar to Wine, then isn't it just as legal as running Windows games in Linux environments?
If that's not the case, this would be technically feasible, but of dubious legality.