What are you saying???
Of course you can...
I own a GT2 and all watchfaces which appear in the Huawei health app under de "mine" tab (Devices/Watch GT2/Watchfaces/Mine) are stored locally in the watch and can be changed on the fly directly from the watch without using the phone...
No you cannot. not with your OWN CUSTOM faces anyway, not without rooting/xposed apparently.
The standard way we are generally talking about here is by going down the chinese id/huawei health app route, and this way you can only install a single watch face of your own. This will then get overwritten when you download another. We are not talking about all the standard watch faces available in the health app of which you can have many on the watch itself