Maybe I misunderstand you, but you can turn on/off hiding the read messages.
If you turn it on then only the unread messages from the selected thread are shown.
I don't see any benefits to show the messages in a view at the right of the threads instead of showing them in a view at the bottom, next to the messages. A view at the right will take too much space from the thread view on smaller screens.
Expanding/collapsing the threads in the upper view is not longer possible which has its pros and cons, but I am afraid it will stay this way.