it is using double ratchet but heres the most recent tob. the fact that they use distributed self hostable relays and no user identifiers is more secure imo
yeah you can only use one device at a time because simplex doesn't have accounts, just devices. but you can make another account per device and add them to your chats
4 and 5, from what I remember you have to opt into being discoverable by number even if you give it your contacts.
that said I don't fw signal for the other reasons stated, it's basically just the easiest secure alternative to texting.
you're right I'm trippin