He's not experienced but neither was Baleba when Brighton got him from Lille. In fact, Brighton got him at 19 and wasn't a full time starter at Lille but he's come on in leaps and bounds in 2 years. I guess what I'm saying is that Amadou Kone can replace Case's role in the squad, which is as a bench player who can rotate with Ugarte. But the upside with him, is that I can see him developing at such a rate that in 2 years he becomes a starter in the first 11. I think him and Sekou Kone can play together or he could play with Mainoo.
However if the club have huge hopes for Sekou that he can come into the squad next season and they want to give him minutes, then we might not need Amadou Kone. I think it all depends on the plans for Sekou and the rate of his development. If they want to ease him in, perhaps give him a loan or two and take their time to build him up, then if we sell Case, I wouldn't hesitate to get Amadou Kone now.
Another young player I like is Ayyoub Bouaddi, althought he's not a Case replacement, more a replacement for Erikksen as a defensive playmaker.
Of the players you mentioned, I only know of Habib Diarra, who's not really a DM. He's even played as a winger, I think he's more a AM as he's got quite good attacking instincts. He's at Strasbourg so means Chelsea will poach him if he's any good.
I will look out for Mukau and Tessman.