@Savid Yup this story lacks depth and there are so many loopholes. But I have my own pov about your questions.
1. I think the old lady with bizarre hair is someone who is selling Haley sextoys. She maybe not directly involved in Mason and Haley's soul exchanging thing.
2. After getting obsessed with Noah, Haley maybe dig about Noah's past and found about Mason then maybe he did some kind of rituals to do something. This part still remain mystery but I think Haley was going to capture Mason's body and atleast have s*x once and then again come back to his body. But as Mason died, his body maybe didn't accept Haley's soul and both got destroyed.
3. Mason is a straight man but the body he is into is a gay body, with continuous sexual activities and drug abused. Just because your soul become different person doesn't mean you body will feel different. If we think back Mason's preference was a cute girl with short appearance where he needs to protect that person. Noah as a child was like that too. Dreams always comes showing subconscious mix up of your daily life and things you're thinking. His body was not getting s*x and his soul wanted someone petite and cute maybe that's why his dreams got twisted. Not sure if I am right but without any other hints I can only made this up as my pov.
4. Mason's brothers role here was nothing, his only family member died which led him to sadness obviously and freedom from any family bound so that can help in the story plot where whatever happens to Mason, nobody is going to dig about him anymore.
If anyone think about any other thing please tell me.