Multi line thread pane. Is that a multi line list so you can get two or three lines of the subject in the list?

Yes, also the date ,contact...

Can you supply an image of what you mean as I'm not fully understanding what you would expect to see

re: Yes, also the date ,contact. because it seems you are asking to see the headers which are already displayed in the list when threaded view is selected provided you have enabled the columns for those fields.

or if you highlight all the threaded section (threaded must be enabled and you click on the thread icon) then you will see the emails one below the other listed as messages in the Message Pane.

or if you right click on an email and choose to view as conversation. You can see all emails in thread and include all the responses from you in a new tab,. Then in new tab you can leave the threaded view enabled or disable it to see all emails listed, then you change the sort by date if you want to see latest/ newest response on top.

or another option: You could try the Conversations addon. Go the link and it will show some images on how this looks. when you select an email, the view in the Message Pane will show the received emails in that thread in the Message pane and you can choose to show details or not. Thunderbird Conversations: https://addons.mozilla.org/en-Us/thunderbird/addon/gmail-conversation-view/?src=userprofile

So there are several options, all doing different views and facilitating various sort arrangements.

I think Bug 441414 refers. Work has recently recommenced, but when it will see the light of day is anybodies guess.