I made some more experiments.
Generally, charging via the touchstone kit seems to work quite stable.
With my old N9+touchstone hack the setup was very sensible with respect to how the phone is put on the charger.
So far, this seems to be no issue with the Jolla phone.

I also measured some charging times.
These values are pretty rough but should serve as an indication of what to expect.
In about 30 minutes the device charged approximately 14% of its capacity (from 43% to 57% absolute charging state).
In about 2 hours and 14 minutes it charged approximately 52% (from 43% to 95%).

By the way, I "modded" the charging unit a little with one of the stickers I got with the phone.
So, now I have my (semi-) official Jolla wireless charging kit.
I attached some photos of this.