Have you installed nova prime?,
If you have, then follow the steps i posted, if it still doesnt work, factory reset your phone and then install nova prime first and then google launcher3 app straight afterwards. This will stop any other apps from drawing over nova and google launcher., plus you will have nova and google now from any screen by saying "google now", also if you install google allo app, then saying "ok google open up google allo app, you will get google assistant. Google assistant is loaded into allo app. Drop me email and let me now how you get on. This works for me, even if i reboot phone it still works. I think when you are selecting nova launcher as default app, huawei home is drawing over that app to set itself to default. Also in app permissions you should select yes for everything for nova (trusted app) and also google launcher3. This will then allow nova and google launcher3 (which is the newer version of google now) to draw over apps you install afterwards.
Please email me and let me know how you get on, i am sure it will work after you do this...,