An existing solution to revert back from the beta
Hi, I hope this doesn't get lost as this isn't a suggestion to change back from the beta, but more-so a post to inform others how to change back to the previous layout.
If you click on the Gear icon on the upper-right, and then scroll down to "View full settings", you will get a bunch of options to make it look like the past layout.
The things I changed to make it look like the old layout were:
* Hide the reading pane
* Hide preview text
* Don't show date headers in message list