For registering, is it possible to use a disposable number? If so, after the number is used for registration, is it ever required again, and exactly is it required to do?
Yes, I use a Google Voice number for signal. It's theoretically possible to use something like a payphone or a free inbound number from callcentric, so long as you can receive incoming calls.
It's not required again, though if someone takes over the number they can re-register on Signal. You'll stop receiving messages then, and your contacts will get a warning that your safety numbers changed.
Wire also got a favorable audit recently. https://techcrunch.com/2017/02/10/messaging-app-wire-now-has...