how do I switch from systemd back to sysv init ?
I dont think you can (at least not easily) at this time. Debian is now tied to systemd. I see a lot of debate on the forums of systemd vs. sysv init. Good points made for both systems. Personally I think systemd is fine. I get much faster boot times. (there were some issues with slow shutdowns and re-boots) but those seem to have been addressed.
As Sparky is based directly on Debian Testing we have to follow their lead (unless Pavroo has other ideas..?..).
Here are some links that might prove of interest..
UselessD (alternative to systemd)
Systemd-to-Sysvinit - converter
(If you try this one, let us know how you made out. I sure would make a full backup first...!!..)
Discussion on Debian Forums
remove, reboot, remove
back to sysv style init
now removed everything else with systemd dependencies
all a stopgap - while I decide : BSD or Gentoo ?