It seems like the phone number is used mainly for matching you up to your contacts, and secondarily used for a first level of authentication. Signal has always encouraged independent verification of folks' public keys for sensitive communication.

Whether or not AWS is risky, I don't think signal has any increased risk hosting their infrastructure on it vs. any other service. The whole point is that comms are end-to-end encrypted from handset to handset, and so any data in Amazon's hands is encrypted.