By the way, I had some (not a lot) experience with QML so if I were to be pointed in the right direction, I *might* be able to submit a patch for this.

(Adding my comment from the "Tree/threaded e-mail list view" ticket here, as it seems on-topic; sorry about cross-posting)

Regarding the mock-up, I feel it's a bit unnatural to add the thread visualisation widget in the e-mail message view. It would work better in the mail list view. Perhaps a git branch visualisation would work better here somehow? Like here:
https://git.occrp.org/libre/property-map/network/master

This would be relatively simple to add, but I wouldn't want to have that as an option in the UI