gabriela,
1. If all three (3) CEs fail, after a timeout period (120 seconds max.) traffic will be forwarded normally as if the CEs were not there. You can use an ACL to block outbound traffic that is not sourced from the CEs. This would prevent traffic that is not redirected from accessing the Internet when all three (3) CEs are down.
2. The hash function is deterministic, therefore both switches would assign IP addresses to the same hash buckets. The "lead" CE is responsible for telling the switches which CEs service which buckets.
3. If I understand your diagram correctly, it looks like you have placed the CEs outside the firewall? This is generally not recommended, as it exposes the CEs unnecessarily to the Internet. It would be better for the CEs to reside inside the firewall.
~Zach