Can you obtain federal aid if you go to a post-bacc program? I've heard many conflicting things about this. The answer to this question is going to be a big factor in whether I choose to stay on as an undergrad for a fifth year, or graduate at the end of this year and then do post-bacc....

You should be available for a Stafford loan for 12 months while completing coursework to be eligible for a degree program (i.e., medical school). However, you should still check with the individual programs for which you are interested. In my opinion the fifth year is a better option.