Marina and her mother will not only host you, they will give you a home. When i booked it, Milene wrote with me already about so many details and took the afraidness of going to Rio down. The flat is very good located, and everything there what you need. Supermarket, food, restaurants, busline. Metro is like 12 mins by feet. I would say it is not a dangerous area. Nothing happened. We also went out for Carnaval with the family and it was a pleasure. If you go to Rio book the flat, it will be the right decision, you will feel like visiting friends! :)
For going to Copacabana or whereever you can take the bus, its safe and 4 reais per ride. Otherwise use uber.