I think that Sora fading away in the ending was the moment he 'dies'. It's been heavily hinted/blatantly stated throughout KH3 that he's burning his life force/heart to save others, so finally rescuing Kairi and bringing her home is what ends it for him.
But since he still has regrets, he ends up in the TWEWY version of Shibuya, probably to be offered another chance with the Reapers Game. Seeing Yozora gives me Joshua vibes, like he's the one running things with the way he's watching from high up.
Riku being there confused me a little, but then again Riku has jumped in after him to bail him out before, maybe he subconsciously followed to be Sora's partner.
As for the person in the black coat, I agree that it might be MoM.
Who MoM is, however, my money is on him being
Demyx's Somebody. After all, it was revealed he has a connection to events before the Keyblade War just like Marluxia, Larxene, and Luxord do, and Demyx is the closest in personality to him.