When I am in this pose I have no clue if my legs are parallel or not. In France a teacher told me that my legs were drifting apart and he recommended to press a block between my thighs. This was difficult, I didn't repeat it.
I made it easier for me today: I took a strap. It kept my legs together. I was glad that I could do the pose, despite the strap around my legs above the knees. This was a great help in understanding the pose.
Breathing: Usually I really exhale as much as I can when I have prepared myself for pushing myself up (I'm already on my head). When the lungs are empty I inhale and imagine that my body is like a balloon and that the air is blowing up my body. I inhale as much as I can and stem myself into the pose. Somehow this helps me.
(It's so difficult to describe in detail what I'm doing. I hope it is still understandable.)