Hey Striped,
Not sure if this is helpful at this point, but I figured I'd shoot you a message anyway.
I'm a PhD student at UChicago. You had some questions about the MAPSS program, so here is what I've observed firsthand: MAPSS is kind of a crapshoot. It's a cash cow, I believe, in the sense that it's a very, very big program and students aren't offered a lot of guidance. A lot of the professors are at the top of their field, or are under a tremendous amount of pressure to move to the top of their field. They are busy with their own research, and they also have their PhD students whom they advise. This means that there is a lot of competition among MAPSS students to stand out. The program is INTENSE. I've heard many students complain about not being able to find a thesis advisor easily. That said, if you are willing and able to put in the work, MAPSS can be a fantastic opportunity. Basically, if you find a professor to work with, and the professor values your work, provides you with research opportunities and/or guidance, and is willing to write you a letter of recommendation at the end of the year when you're applying to PhD programs, you have yourself the golden ticket. You are also required to write a thesis, which provides you with a solid writing sample for your PhD apps. And you'll learn a lot and glean a lot of insight into what a PhD program will be like. But you've got to want it. The program is rough. A lot of people in the SS PhD programs had MA degrees coming into the programs, but not all. As far as I can tell, those who didn't have an MA had significant research experience.