“Here is the problem: My high school will only accept one AP class from the offerings my online school provides (through a university) so I’ve been taking mostly regular classes. Hence the lower than competitive weighted GPA and rank.”
I’m not a Brown AO, but I interact with them from time to time. Simply explain your situation as concisely as possible in a letter/supplement of some sort. The AO will read it, “get it”, and proceed from there. They are used to dealing with special circumstances of all sorts in making their holistic evaluations of an applicant’s file – deciding whether it’s someone they want to want to make a case for when they all meet. Your chances aren’t great (said simply because no one’s are, despite what they might think), but you do have one : )