According to the current version of the iPhone app, yes, my group chats are E2E encrypted.
I believe the mechanism is to use a generic / per message key to encrypt each individual message, and then use each recipient's public key to encrypt the encryption key before sending that (along with a link to the cipher text) to each end user.
This is also why sending a video the first time takes forever (encrypt, upload, encrypt key, send key, send link), while forwarding it to another person (encrypt key, send key, send link) does not.
Yes. It's actually one of the strongest points of whatsapp compared to other mainstream messanging services (Though Telegram is often touted as a more trustworthly alternative, it does not E2E encrypt by default and in my experience that means almost no-one does it). They've stuck with it even though it results in somewhat goofy UX (for example, no server-side logs, you can't log into other devices without your phone being online, etc).
Yes they are supposed to be, when a user joins a chat they cannot see prior messages and I believe this is a byproduct of their key being added to the ring.
