Why would they discriminate against you by rejecting you now if they would WL you later in the cycle? All schools need to build a WL, even if they aren't going to use it (and pretty much all schools do use them to one degree or another), so why wouldn't they make everyone eligible to be placed on it, regardless of when in the cycle they interview?
You aren't R because you are clearly good enough to be admitted. You aren't A because they are not going to fill the class with people good enough to be admitted before they have had a chance to interview everyone given an II throughout the course of the cycle, and you apparently did not score as highly as whoever made the cut now.
It turns out that they really do hold back spots so that everyone who interviews has a shot at being accepted. Interviewing early really only means that they liked you more when they sent out invitations, but it really doesn't otherwise give anyone an inherent advantage to be admitted over people interviewing later, just like the adcoms have been telling us all along.
It sucks that you didn't receive an A, but you are clearly highly thought of, since you were interviewed so early, and you weren't rejected, so you're still in the game. Not as good as it could be, but also definitely not as bad either. Just try to hang in there; it's going to be a long cycle.
Any advice on how to go about bettering my chances of getting off the waitlist? I’ve heard that sending a letter of intent, grade updates (I won’t have any), as well as other application updates can help. How true is this? Also, regarding the LOI when is a good time to send that? Thanks!