Dear J.,
Did the doctor take a urine sample? Maybe she has a low bladder infection. Also, maybe you could teach her how to strengthen her muscles down there, it is called a Kegle Exercise, I think. It seems to me that it would be hard to explain to a child, but go on the internet with her sitting on your lap, and get the instructions. She will feel involved in solving the problem and important.
At this point, I think that the more the two of you work together, and call it that to her. I know that you have tried a lot of together time, now work on this problem together, using those words, because you do not want her to be embarrassed at school. Use those words too. sing "The more we work together together together the happier we will be.? You know that old song.
You have done all the right things with the new baby coming, and I don't feel that has anything to do with this. Maybe she just does get busy and doesn't want to stop and go to the bathroom. Maybe rewards for not peeing in her pants would help. Not big ones, but maybe stickers, don't kids love stickers? For a whole afternoon, or morning not peeing in her pants? And no stickers when she does pee in her panties. Do you have a small stool so that she can get on the potty easily by herself? Are her clothes easy to get up and down? We forget how hard it is to be a little kid and have to learn all of this stuff.
Good Luck, I do know how frustrated you must be. My gr grand daughter did that too, for a long time after she was potty trained. I think that she just got busy in addition to being hard headed - she is still strong minded. She says that she can now swim and ride her 'bicycle' and when she is 5 years old she will learn to drive a car - with mother in the back ground yelling 'not a real car!'.
Peace, C. N.