I just re-installed the demo version in order to leave a review. I'll uninstall it later. Presumably it won't upset my existing CC installation! I guess if I don't open it - it can't.
Re the google+ thing, I've never seen this message. I do have a gmail account which automatically has a google+ account, but I can't see any of my app reviews on it. Maybe I'm just not skilled enough in social media use
Anyway, good luck on your quest to raise the rating. I understand your reasoning. The problem is certainly that those who understand the app and like it go to the full version very quickly and rate that. Those who don't leave low reviews, probably because they can't understand it.
Maybe you could use an in-app payment rather than two versions.