I successfully downgraded my 2DS from 10.3 using tuxsh sysupdater. Everything works fine in sysnand but browserhax boot rate is poor. CakesCFW boots fine. The only way for me to install an unsigned cia in emunand was to inject a cia installer into the health and safety app. Even with that, the only unsigned cia I installed (hblauncher_loader) needed a full emunand reboot to show properly in menu, before that it stayed gray. FBI doesn't have any privileges in emunand if launched from sliderhax(it can't even delete cia files in SD).
EDIT: Currently Updating emunand to 10.3. Let's see where this leads to.