> Why do you trust Google more than Facebook? [1]
How is using Google for domain fronting for the purpose of censorship circumvention "trusting Google"? They don't get to see the message content (just like Facebook doesn't see the content for WhatsApp).
> Did you also verify that no untrusted third party receives your backed up private keys?
Private keys are not backed up in either service, as far as I know.
> Did you check the source code at signal's android/ios client repo?
Given the lack of reproducible builds on iOS/the app store, any source code audit is pointless if the app vendor is included in your threat model.