You can autostart applications on boot by listening to the android.intent.action.BOOT_COMPLETED
intent in a BroadcastReceiver and start your Activity from there. In the Activity you can register yourself as the new default homescreen[1] and handle the keys.
I think there are some instances that you can’t handle without modifying the framework (like longpress on Home to show currently active Applications) – I could also be mistaken though.
But for a prototype that could be sufficient.
Have fun tinkering!
[1]:
<intent-filter>
<action android:name="android.intent.action.MAIN" />
<category android:name="android.intent.category.HOME" />
<category android:name="android.intent.category.DEFAULT" />
</intent-filter>