You can run niri from within other DEs, but I'm unsure whether it works well for your use-case. From the docs[0]:

"You can also run niri inside an existing desktop session. Then it will open as a window, where you can give it a try. Note that this windowed mode is mainly meant for development, so it is a bit buggy (in particular, there are issues with hotkeys)."

IIRC in this case the Mod key is by default Alt instead of Super.

[0] https://yalter.github.io/niri/Getting-Started.html

I'm not sure, but I doubt it. You could try PaperWM [0] inside Gnome to get a feel for the scrolling WM workflow, and see if it's worth switching to niri proper for you.

[0] https://github.com/paperwm/PaperWM