In GW 2018 sp1 they made some improvements with controlling this feature.
In GW Admin, go to your PO object, Client Options, General, you can uncheck (enable conversation threading).
If you already have a client with it turned on, right click in the toolbar section,
edit display settings, Edit View by = Details.
You can then uncheck any other items you don't like or hit the GW2104 defaults button.
They still have not enabled you to pick the order that the threads appear, but it
might be in an upcoming release ( I hope).
The display needs to default to newest on top of the thread.
You need to be able to control this feature from the default client options in the PO and not
per individual client. We want to deploy with this feature turned off and add it only when requested by
Wishing a lot more control on Thread-Layout!
From a UX perspective it is currently misleading. The initial message is the most prominent one. Mails within the discussion thread - and even the latest mail - is displayed very small.
I'm not able to show Mails within a thread in a full view (as the initial mail is shown). The only way is to open it within a new window, which is bad.