This link may help: https://support.mozilla.org/en-US/kb/message-threading-thunderbird and there is an addon, 'thunderbird conversations' that you may wish to pursue.

Thank you, I'll check it out and come back to mark this as solved if applicable.

re :2. The DESCENDING view does this, which I feel is confusing: a) Single messages and thread topics in the selected folder are sorted descending. For threads, it weirdly uses the timestamp of the FIRST message in the thread instead of the most recent message. b) Messages within a thread are sorted ascending instead of descending.

That is because the 'Sort by' only applies to top level emails, so if you use 'unthreaded' then all emails are single top level emails and all emails will get the 'Sort by' option applied. If you use 'Threaded', then the 'Sort by' gets applied to the initial email and not it's sub threads/replies. Those emails are treated as a conversation, so each email is appended to the initial email meaning the newest will always get appended to the bottom. If you have ever used eg: Facebook messaging chat then you will notice new comments are below the previous / appended to the conversation.

This issue has often been asked int he Bugzilla forum and there seems to be no interest in altering the current set up. However, if you choose to right click on initial email in thread and select 'Open' > 'Open Message in conversation' then the entire thread is opened in a new tab. You can then sort by date if required. It means it's much easier and less cluttered to just see everything to do with that conversation including any replies you have made. Try it out :)

Helpful information, thank you for the tip, and thank you both for your replies. It is surprising to me that there seems to be no interest in making this work like Outlook, as that feels like the most 'natural' way to deal with threaded messages. I'll mark this as closed.