What an interesting topic!
You're definitely right to point out some of the dangers of positivity if it isn't harnessed well.
I would add to your list, expanding on your point that 'positivity can be a means to sugarcoat': willful blindness. It is possible to conjure up a positive attitude towards something that really shouldn't be treated as such. Positive illusions can actually prevent pragmatic action or change in some cases. It can also be a means of repression (by erroneously forcing a positive view).
When I got into mindfulness, for example, 90% of it had a wonderful impact on my life, but that 10% that wasn't great was exaggerated optimism, occasional repression (like rather than getting angry, trying to embody compassion) and willful blindness.
Sometimes we have to confront the ugly, nasty aspects of reality; that can even be a pathway to healing. In learning assertiveness, I had to actually discard some positive views so that I could grapple more effectively with the darker aspects of human nature. I was so optimistic and positive that I was basically blind to the little hierarchical, power-driven behaviors that underlie many peoples behavior. To become assertive I had to first descend into the muck, so to speak.
But I absolutely agree with you that over-all being positive, and thinking positively, is powerful and good. The good far out weighs the bad!
Thanks for making such an intriguing post :)