Would you happen to know if you can send stuff using Signal to someone whose phone is offline? You do seem to be able to do it using Wire but I would rather avoid it as the device verification seems broken.
Sure. Messaging via Signal (or via any other modern mobile messaging app) is designed to be asynchronous and doesn't require all participants of a chat to be online at the same time.
Signal has perfect forward secrecy though. I am unsure how that could be achieved if phone 1 sends a message, the phone gets disconnected and then phone 2 connects to it.