Aussies cannot participate in the United CPP unless they find themselves a greencard. The difference when compared to an AA wholly owned flow is that the CPP doesn't contractually guarantee you a job.
Re: the potential for E3s to not be renewed. It is possible that it might happen, but once an Aussie is on property they cost no more than a US pilot. The renewal is simply a letter, and the Aussie pays the ~$150 fee at the consulate.
Personally I don't think the music will stop anytime soon, all you have to do is look at the mandatory retirements on the APC site. I heard from a buddy that AA will hire 1000 pilots next year.