I used a universal mod for Renpy called 0x52_URM, you place it in the game folder of the game and when you are playing you go to your apartment and press alt+M, in the menu it should say labels and in search write bella, look for the option that It says "cheap_apartament_bella_event and you press JUMP that forces the game to appear bella, and thus use it for other scenes since this thing about it being random is bullshit. I recommend Use it only for Bella, although you can do the same with other characters but always respecting that you have that mission or you can break the game. I used it for Bella to come to the apartment and also for Nestor to appear at the bank.
If you provide me with your save, I could do it for you, but later you will need Bella to return to the apartment again, so it is better that you do it yourself.