p.s. this is what I plan to implement for forks / nested replies : %Ff29qCa...

It was discussed a bunch with a good number of people. Patchwork currently does nested replies by using the branch property - which feels like a hack which uses a property I think should only be for causal determination to manually move a message into position by "faking" it's causal position. This works ok but I personally think is a muddling of concerns.