Mick would be a decent second driver by taking Hamilton's place and he wouldn't be much trouble for Russell I think. Russell would really like to have a teammate like him as Russell is very competitive and hungry for wins.
We don't know this.We don't know how Mick will behave in a winning car,he has not been in a better car than Haas which has been the best for him,differently Magnussen has been before in better teams than Haas and he of course has much more experience than Mick Schumacher.I think we should wait to see Mick in a Mercedes,in a winning car as I believe even drivers not up to the level of talent of Hamilton and Verstappen have won races with that car and Nico Rosberg even the Championship season of 2016 if I remember it correctly.