This may have been answered but my issue seems to happen upon restart, the ads go away on the lockscreen but after a restart the appear back, and nova launcher wont set as default either though I was able to get the play store on it but the fire UI still seems to be there.
As for Nova Launcher not setting as default you are probably on FireOS 18.104.22.168 if you cant change the default launcher, you might be able change the navigation bar to a app like this
And have it set to open Nova instead of the stock launcher.Not exactly what you asked I know but this one works pretty.
Swipe the edges of your device to do many useful actions.play.google.com
I've set Swipe from RightEdge -> Back
Swipe from Left edge -> Google Now
Swipe Up -> Start Lawnchair
Far Swipe Up -> Recents