How are shift+enter newlines "totally against the grain of the system design"? Most every chat system I'm aware of supports this and displays it in a sane way. If people are misusing the system, training/instruction is in order.
I still fear the web UI. I guess emailing out bullet lists was against the implicit use case the gmail UI team had in mind.
Stuff like this definitely changes how tools are used.
Also, when replying to a long mail thread, showing you the previous list of recipients and requiring you to select those that you want to include in your next mail. Could limit CC sprawl quite a bit.
This way I don't have to worry about hitting Enter at the wrong time. I can also preview the message so I see exactly how it will be formatted, and no one has to see the "mike is typing" messages while I edit.
I also went to the extreme of creating a private Slack team just for myself. I was using the personal channel trick and found that if I included an image in my talking-to-myself message, and then pasted it to another channel, it didn't automatically display the image in that channel. I guess Slack figured the image had already been displayed somewhere within the team even though it wasn't in a public or private channel. I have a feeling this has been fixed by now.