I'd say it's the best selling Switch game by now.
Hard to say. Breath of the Wild was at 4.70 million according to their reports by September 30th. I have no doubt Odyssey is easily 3 million +, probably over 4 by now with Black Friday and all but Zelda would most likely have sold another chunk as well with the holidays.
Optimistic side I'd say Mario is closing in on 5 million. Zelda should be at 5.5 or so by now? Possibly more, I'm never good at predicting :D