Three things come to mind.
Perhaps the most important thing is that it sounds like you are doing very well. You are a very compelling and very competitive applicant to any university. I think that you just need to keep doing what you are already doing.
Secondly, other than maybe winning a Nobel Prize or having a parent who is the popular leader of a medium sized country, I do not think that there is anything that you can do to ensure that you get accepted to a school with a 13% acceptance rate. I think that you just need to do your best and keep an open mind. There are a LOT of very good universities in the USA. WUSTL is one of them (and I have visited it), but there are a lot of other very good universities also.
My last thought is with regard to double majoring in molecular biology and fine arts. I do not think that you will need a double major. You can just major in molecular biology and take the fine arts classes. Anything that you do with a double major, you can do with the degree in biology plus having taken the fine art classes.
I think that the point of ECs is to show responsible behavior, the ability to focus on an area and excel in that area, and the ability to interact with other people. I do not think that ECs need to be related to your potential major. My ECs were not related to my major, and this did not cause any problems way back when I was applying to a few highly ranked schools.
I think that you are doing very well, and you just need to keep doing very well. When the time comes to apply to universities, keep an open mind, keep your budget in mind, and apply to at least one and preferably at least two safeties.