I think Werner up top would give a better chance of success, especially because the whole team could sit a bit deeper to possibly offer Werner space in behind. Also if he's running at a defence it's inevitable that space between the lines would open up. Bigger problem there is Werner can't really hold the ball up and his contributions to our buildup play have been very poor thus far.
Re: the 4231, at this point it feels like that's what we might just have to use, especially if Kante's forward-minded instincts can't be curbed. If he can be convinced to just sit deep and sweep laterally (as he was doing pretty well at the start of the year), then I think a 4231 is a good option, shifting to more of a 4222 in possession (Havertz moving forward from the 10 to partner Werner). That gets Havertz closer to goal and gives Werner someone to play off of, which for me is probably the way to get the best out of both of them. Then you have Kante sitting deep and sweeping, with Mount probably stepping up into the 10 position when we have the ball.