Thanks for your project. I don't dive in cryptography much these days.
Of course E2E encryption helps privacy. I was more thinking that it doesn't provide anonymity since the server knows who speaks to who but it doesn't know the content.
The way the memo would be used, looking at the way it was implemented, would be for one time message or a small number of messages. Take our conversation: I wrote, you answered and I am answering back. If you mix the order it is not the same conversation any more. As for encrypting deterministically, you don't want people to spot patterns.
Why not use it for IM? A certain number of reasons. One is that you can create a message and say that I sent it to you(I didn't see any signing of the message so using transfer or comment on the blockchain fixes that). There are some technical reasons why you might not want to use the same key for too many messages (I need to check more the protocol used).
I think I will open a bug issue about the encryption being deterministic. I don't see a reason why it would be that way.