One Sum Calculation not working for one dimension

I can see that your legacy model is seriously very far from ideal. Other than many-to-many, I can see a lot of bidirectional relationships which themselves can cause unexpected results. If I were in your shoes, I would just start rebuilding this model - I'd combine tables with the same structures into one, use a separate dates table (like you mentioned in the video), use dim tables to bridge the fact tables with different structures.