I visited Columbia on Thursday and Friday. Regarding the first point above, the DGS told me they admitted over twice as many applicants as they want. Considering a lot of people I had met were weighing offers from various top 10 schools, I suppose this is typical.
As for the second point, he told me not to expect any movement on the wait list until April 15. And he recommended waiting til the very last hour, which happens to be 5 pm on April 15.
Also, Stanford's prospective students event is on Monday/Tuesday, and I heard another top school has theirs in the middle of the week, though I don't recall which school. So yes, I wouldn't expect people with offers from top places to make decisions until very close to the deadline.