Please let me know if you find
The REST-Api Key you find on your G3-User Page. Login to g3 and klick on your nam at the top-right (using default theme).
halinasplace is right. I thought I solved this issue in 0.8.3 .. but now again :)
There are still some issues about updating the caches. If you have unexpectable behavior safe WPG3 Options page at least 2 times. It might help.
Thanks for your comments. I'll see if I can keep my promisses to enable integration in posts too in next release.
I'm heavily working on the Insert-Image (imagechose) and an additional (default)-Inline-block-template now.