Called the office today to see if I could get more info on waitlist movement and here's what I was told:
-Most movement is from the top third, but they can pull people from the other tiers; it's rare but has happened before in the past, and the lady kindly told me not to get my hopes up as someone who is in the middle third.
-They look at demographics (male/female, IS/OOS) but also at exposure and experiences when looking for someone to replace the declined acceptance because they want to maintain the diversity--of course, only the adcoms know the specifics but this was the most that the lady could tell me since she didn't know all the details.
-The do not work strictly from the top of the waitlist to the bottom, but of course, you are more likely to be pulled off if you're in the top third. And again, this is because of demographics.
-If you send in updates/letters of interest, they are not guaranteed to be read UNLESS you are being considered from the waitlist ie. if there is a pool of candidates that are likely to be pulled from the waitlist, they might look at the updates/letters of interest and take that into consideration. This is due to the fact that there are just so many people on the waitlist that it would be impossible for the adcoms to try to read through every single update/LoI that is received.