My wife is used to FLauncher and will probably get annoyed if I change the Shield's interface (again...) but maybe I'll try it out on the Google TV Chromecast I have for holiday trips.
I just hope this is still possible on the new one, I can see a future where they make it impossible but hopefully not yet.
I agree that it's a good upgrade too, the only dowside is that it's not portable anymore, I used to take it with me on any travel to have my own home theater set up like I like it with me when I was living in Hotels and AirBnB's for a year on business travel. Especially in Asia the TVs are in the local language and I never have any idea how to change the language from Korean or Japanese to English so I can use the TV. But putting in the dongle and turning on the TV was always possible.
I doubt they'd block alternate launchers as they seem happy enough hosting them on Google Play.
Side-loading will be safe as Android being open is its whole deal. The EU are forcing Apple to be more open so there's no way Google wants a target on their back.