I think what you’re suggesting, adding two substantial Thorin-centric prequel sequences into M4 The Book Edit might possibly undermine Bilbo’s story rather than improve it.

I haven’t seen that, no. But, on top of the explanations for reinserting those Erebor and Moria pieces I’ve already given - Thorin may not be the main character and the story of ‘The Hobbit’ is not his journey, he is still a very important character in everything. I’ve always seen him as a deuteragonist in the story since so much revolves around him: his kingdom, his throne, the Arkenstone which is now ‘his’ as the king’s jewel, Smaug is now his archnemesis as the usurper of his kingdom and its wealth etc.

Reinserting the Erebor and Moria sequences just helps to add weight to the character and really stress his importance and show him as the king he is - working to help his people when they’re displaced, trying to set up another kingdom for them and earning the right of kingship in the eyes of others on the battlefield. Hopefully not to the detriment of Bilbo directly, but just to show Thorin as the kind of man (or Dwarf) he is and show him as the power-player in the story that he is, that’s all.