With regard to how vulnerable E3s may be to being let go, the only real vulnerability is if your visa is coming up for renewal, the company could simply choose to let it lapse. If you've still got a good amount of time left to run, and you're up your company's list a little way, you're probably as safe as those around you. The other factor is that if you're on an E3 and getting close to upgrade (which a lot are now), your company will more than likely want to keep you, as there is a shortage of captains through a lot of regionals right now.
Despite a reduction in hours for April, we are still hiring FOs for the foreseeable future.