Firstly, i want to clarify i have no issues with 4-3-1-2. I actually love it and i had argued many times last season we should give it a try as the squad was perfect for it. But i can't see it working with this season squad.
I don't think LCM would be a problem. We have Vidal and Perisic, who had looked good as LCM in the only time Spalletti gave a try to diamond. The issues would be fullbacks and, most importantly, AM.
If we switch to 4-3-1-2 we need to forget about starting both Dumfries and Dimarco in fullbacks. If we start an attacking fullback then we need to have a defensive fullback on the other side who will stay behind and basically become a third cb. If Maicon and Maxwell couldn't make the two attacking fullbacks combo work there's no chance it will be any different with Dumfries and Dimarco. So it's either Darmian/D'Ambrosio-Dimarco or Dumfries-Bastoni. I would go for the former. The attacking fullback, on the contrary with the wingback now, isn't going to have lots of space so we would need someone who's good at crossing and that's one of Dumfries biggest weaknesses. But i have doubts if even the Darmian-Dimarco duo could make it work. The attacking fullback needs to contribute a lot in attack as we will have no wing play.
But the main issue is AM. We don't just need someone who can play in the position. We aren't going to have attacking play on the wings so the AM needs to be a playmaker with great passing and vision in order to break defences and i haven't seen these things from Hakan. We have been there in the past. We had tried many players in the position with Stankovic being the best but even he struggled and we had to count on forwards individual brilliance to score. Only when we signed a world class playmaker in Sneijder diamond finally worked. The only one who could pull it off is Alexis but he's too old to start week in week out and i fear we would miss a lot in defensive aspect.