Bills are usually split on a percentage basis, except in 'room-mate' situations. He sounds like a room-mate with great benefits to HIM. You, not so much.

The dog is 2 and won't change. It was bred to be 'reactive'. Sometimes, you can train that out of a dog. but at 2 years, she's set in her ways. How many people has she bitten? And lunges at children?! I don't know where you live, but here, cops or guns would have been involved.

This won't end well. She'll hurt/kill 1 of your cats or bite someone(probably you) badly.