1) Taken to the extreme, my understanding is that the "collaborative partner" can actually be an anonymous person on the Internet. Think: open source project on GitHub that you want to extend or add to. In a collaborative situation for the task you are required to highlight and write about elements that you specifically added to the project and give attribution to parts created by others. Fortunately or unfortunately this leaves the door open to getting "professional help". Of course the same risks are true for any AP performance task - in AP Studio Art, little Johnny Picasso might have an edge. The checks against cheating like this are two: 1) the teacher must attest that the student's claims about what they contributed are true and that the student's writing is his/her own 2) the AP reader is allowed to use their judgement to ascertain the quality/veracity of the student's contribution.
I can tell you from 15 years of experience reading students' code in AP CS A that you can tell - even if you've never met the student - when something is amiss. Writing code is not like solving a math problem - it is a form of personal expression and you can just tell when you're reading a pro v. a student. Not that a student in this class couldn't write like a pro, but your spidey senses start to tingle and you begin to read with a suspicious eye. Frequently, the written prose is a giveaway that the student doesn't really know what they did. This may be small consolation, but I actually doubt instances of cheating like this will be few and far between, and I trust the AP readers to suss out the quality of the student's contribution pretty well.
2) Brook and I are getting back on the podcast horse next week, and our first guest is Crystal Furman from the College Board who is the woman "in charge" of many assessment matters. So we'll ask!