I'm sorry to hear about your fish all the other problems you're facing with your other bettas (and your rat). My betta just passed away a few days ago, I did bury him in my garden, although I didn't prepare a box. I did love him though he was my fist fish, I know its sad I'm sorry you lost your fish
Also, I have 2 rats, have had 2 prior that both died of tumors/old age. I really hope your rat pulls through, they are wonderful pets even if a large majority of people can't get past the fact they are rats. Best wishes to you and your pets.