My recruiter said that for the summer, most people were there for June, July, and part of August. Not much getting there in May and leaving mid August.
For new people that would be 1 week of moving in, Discovery Day, Traditions, then training. That leaves about 8-10 weeks of work before they leave and new people have to be trained. They said it wasn't worth having people for such a short time.
For alumni, my recruiter said that most wanted different locations and different types of jobs.
Alumni can go back with housing, but you either have to transfer (and be allowed) or go back to your old work location. You can get housing for 4 weeks at a time, when your 4 weeks are nearing completion, you can get housing for another 4 weeks, and then again and again if room is available. My recruiter said they were talking about having a set number for alumni and that it would be available for as long as you wanted.
The only thing that the freeze may effect is the number of hours you get. Instead of 40 (that many CPs get), you may end up getting 30-35 a week.