I'm not wildly happy about adding more OS-specific code to tmux for something it already does perfectly well everywhere (including Linux right now).
If it has to be done differently then - glibc has the daemon() function, and tmux uses it on Linux. Shouldn't this code be part of glibc instead of tmux?
There doesn't seem to be any discussion following these additional questions from the tmux side. So what you're really saying is that systemd has been aware of the architectural issues with their design for five years, yet have not bothered to fix their end?