Hi,
I actually figured out that they didn't accept it because my faculty forgot to put his contact information. That was just the automated system sending out the email which caused confusion. I asked my recommender to resend it and I also asked one of my bio professors and he said he would write it for me in case they don't accept the other letter.
1) I just checked the list of acceptable sciences. Did not think each university would have different sets of subjects accepted tbh.
2) Here is the list for WESTERNU
not sure about how they decide on that.
3) To answer your question, I go to UCLA so classes are well over 100 people so when I asked a few of my bio professors back when I was gearing up for my application they said they already reached their capacity to write a letter this cycle so I asked my chem and physics who knew me very well and I knew they would write a better letter instead. It is very difficult to stand out when many students are already excellent at UCLA. I had to pick and choose to be able to build good relationships and be exceptional in a few of my science courses in a pool of great and smart students to even get a letter especially due to the loads of studying at the time.
BCP GPA: 3.76
Thank you so much for helping