the problem with comparing this to P8P adventures is that those are on some level meant to leave space for the DM. They can't encompass everything and aren't truly complete until the players (including the GM) meet it and fill it. Meanwhile BG3 is an absolute shambles of a story that never actually holds together when it's a game that doesn't have the same degree of openness to allow players to truly mold it.
I understand that expectations for the story of a video game are higher. Coming from P&P, I am just so used to this that pixies start shooting material for the spin-offs the moment the camera pans away. I should add, BG3 is my first video RPG game since the first Gothic or KotOR 2. Back when a buddy and me were playing Diablo 2, we used to enact how our characters went into the equipment shop in town and got the name of the owner wrong differently every time. Yes, I suppose we're a weird bunch.
I keep getting the impression that Mephistopheles did not necessarily know about the theft,
The aforementioned pixies are probably also to blame for me thinking that the Crown of Karsus was stolen from Mammon's (Helsik's patron) vault on Minauros who had just forgotten about the crown among his many other riches. Since Rafael and Durge stole it from Mephistopheles, perhaps he merely seized the opportunity. From the speech Rafael gives at Sharess' Caress about the crown, we know he tried to get it while it was still on Toril. Perhaps his father succeeded where he failed and Rafael felt sidelined. As a half-blood/Cambion he had no chance to ascend in station like a "true" devil in the old cosmology. The crown was his chance to break out of the rat race. The question remains why Mephistopheles never used the crown.
I have been trying to find the link for so long regret not bookmarking the moment I watched it, but there was a (?dev?) interview, supposedly we could potentially have revisited the hells more specifically Vlaakith's temple grounds which perhaps would have fleshed out more of the detail we are having on this topic maybe (?) never came to be beyond the stage of concepting I assume a lack of time was probably the biggest reason to it, that subject included a lot more that also didn't make it to the game for reasons such as not matching the flow of the game and lack of proper tech to have other details work practically from a gameplay perspective regarding other stuff, like something that could take place between Act 2 and Act 3 for example.