Ooops, sorry, I should have updated or linked to this message. Actually, it is a lot simpler, and I added a short paragraph in the FAQ a while ago based on the suggestions provided in the long thread about issue 831 ( https://github.com/syncthing/syncthing/issues/831):
Basically, just leave things under Android/data/com.nutomic.syncthingandroid. No root needed.
(Now, that did work with the official ST android client, but it is also working with the new silk client. No idea why. I’ll ask this in the proper place: Unofficial OpenSilk Android Application).