What a horrible situation, and you have our sympathies on the loss of the other dog. No wonder your surviving girl is traumatized, and I bet you are as well. Does she have any particular treat or games she enjoys? I am sure the back yard smells different, too, now.

Walking her, it sounds like you need to do some obedience training and work with desensitizing her to cars. Try, when you see a car approaching, to have her stop, sit, down, do paw - any series of commands that will keep her focus on you, and relaxed until the car passes. Once the car is gone, continue your walk, still stopping sometimes to run her through her pace, just for fun.

The pooping in the house is a bigger concern. Do you clean the spot thoroughly with a good enzymatic cleaner after she has gone? You need to retrain her to go outside, much like you would with a brand new pup. This will take time, and I know trying to outstubborn a Newf is no easy task. But after she eats, when you know it is about time for her to poop, take her out for a walk, and do not go back inside until she has done so. When she does either outside, praise, praise, praise!

I would concentrate on these two things for now, and eventually, once life is better for everyone, work on getting her comfortable in the back yard again. (Bribery - whether treats or toys are her preference - is a good idea.)

Hope this all helps! Keeping yourself as relaxed as possible on walks - even if it is purely an act - will help, as we all know our nerves travel right down the leash!