Pogba has worked with Kante in pivots multiple times in a 4231. Never lost a match. Just because Fred can't do it, doesnt mean it's impossible. Matic did it very well during the restart last season actually.
Only if you want to start playing McTominay on the wing. That's the equivalent of how Kante and Pogba play together, as they always have a third midfielder in there.
When Pogba starts for France they have only used two midfielders in 4 of the last 29 matches. The opposition in those matches were all outside the top 50 ranked nations, with two of them being outside the top 100. That means in the other 25 matches (and in EVERY match they've played decent opposition) they always have a third midfielder with them, either in the midfield or nominally named out wide but obviously helping out the midfield a lot. It's obviously significantly different than us playing the likes of Rashford, Greenwood and now Sancho, all of whom are attacking forwards who don't do much defensively or help the midfield control the game (hopefully Sancho will be an improvement in the latter regard though).
It's worth noting that during that same period of time there were 16 matches that Pogba didn't start (mostly due to injury). France were much more likely to only use two midfielders in these matches, with 9 matches.
When Pogba starts = 86% of the time they had a third midfielder.
When Pogba does not start = 43% of the time they had a third midfielder.
It's not a perfect comparison as a couple of those matches without him were played with three central defenders and two wingbacks which obviously changes things a fair bit. But they are still much more likely to play two midfielders and two attacking wingers (ie. the same way we play) when Pogba doesn't play compared to when he does. So why do we expect our midfielders to be able to partner Pogba by themselves in the faster and better quality league?
As for the matches after the restart, every team we faced was in poor form and yet we still only played well in the first three matches. After that we just stumbled along and managed to beat most teams with individual quality, with regular complaints in here about our midfield not actually doing their job.