I am just so surprised your Muscovy drake is so aggressive at his age. I have had many Muscovy drakes and have never seen abuse like that. I am not sure you can ever put him back with your females. How many females do you have total. Would you consider rehoming him and maybe trying another Muscovy drake? Or just keep your females and no drake. It's going to take a long time for Bella to get over what she has been through. Lots of good feed and TLC and hopefully she'll be fine.