With Google pay you don't need to swipe up to use it. You just unlock your phone and tap. So if you wanted to use Google pay you would have to download it. As for the Samsung pay you can either uninstall it, or just set Google pay to your default.
Yes you don't need to open the Google app. Just unlock phone and tap. As for the watch 4 I haven't tried it on the watch. I have read that it isn't working on the watch yet. You can install the app, put your card details in, but it's not working if you go to use it. So hopefully that will be fixed soon.