I'll try to cover a few of the questions I know the answer to.
They send out invites for ~150 per interview session. 4 interview sessions = 600 interviews regardless of the number of applications received. The 585 would have been due to last minute cancellations where a replacement invite could not be mailed.
Last year they said there were (I don't remember the exact number) ~1800 applications making it a 1 interview for every 3 applicants.
I don't know how many TAPPers we have, but I don't think it's even close to 100 since the number of USC undergrads we have in our class is <50. I believe the number is <20 (16 if I remember right). If that's the case (and I'm pretty sure it is) that leaves at least 150 open spots or 25% of interviewees who can be accepted.
The info session was open to all, but I know a few of them were pre-pharmers from Pasadena City college. I took two of them and their advisor on the tour. The funny part is, the two we had in our tour group were pre-meds, not pre-pharm.
Here's the info session schedule for those interested.
http://www.usc.edu/schools/pharmacy/pharmd/admission/tour.html
The tours are given by us pharm students, so it may be a good time to ask questions if you think you'll be interviewing and want some background info you can impress you interviewers with.
I don't know the due date for the 300 words, but it should be on your acceptance letter or in the info that comes with it. I could take a look at mine, if I can find it again, but I'm pretty sure it's 1 week or something like that.