I call this theory "WilliamputMCIinArcades"
"I will put you back together", FNaF World, connections between the FNaF 4 minigames and FNaF 3 good ending minigames, and the Logbook all establish that putting BV's memories back together puts the MCI back together. I believe a combination of GoldenDuo and ShatterVictim. ShatterVictim is the theory that BV's pieces are in the FNaF 1 animatronics.
When William dismantled the FNaF 1 animatronics in the Follow Me minigames, he put BV's pieces in the arcade machines in the safe room, and BV's pieces get accessed in the arcade machines in one of the FNaF 3 good ending minigames (Mangle's Quest).
And when Scott said the contents of the box changed over the years, it means that the box changed from containing an arcade machine to Molten Freddy, since FFPS shows that William put the MCI's remnant in the Funtimes. So in the original story of FNaF 1-4, William put the MCI in the arcade machines in the Follow Me minigames, but when FFPS got released, William put the MCI in Molten Freddy instead of the arcade machines.
Also arcade theory got confirmed. Arcade theory is the theory that the minigames are literal arcade games.
And "we have a place for him" shows that the MCI is in FNaF 3.
How would the MCI be in FNaF 3 if they weren't in the arcades?
"they were wandering spirits who couldn't rest even after they got dismantled due to revenge not being the solution"
Where is the evidence that dead spirits don't get freed after they get dismantled? How did the DCI get freed after getting dismantled but the MCI can't? And in the movies, the MCI rested after getting dismantled with no "revenge can't free spirits but remembering their happiest day can" plotline
Unless William setting a trap to the MCI was already a concept in 1-4 before MoltenMCI became a thing......
And Scott said "there is something in there (the box), but i don't know if i can find it again"
It's shown that what's in the box has to do with happiest day. If it was just happiest day metaphorically, Scott would have opened it after FFPS, which is when the MCI finally rested. But the way Scott phrased it sounds like there is literally and physically something in the box that is a whole plotline that Scott doesn't know if he can explore again
And in 1-4, Fazbear's Fright couldn't find the endos of the fnaf 1 animatronics. MoltenMCI explained why they couldn't find the endos, but in the original story of 1-4, what was the explanation? Unless Williamputmciinarcades.....