Can one achieve 100% ? What's the highest that any one has attained?
No, one cannot achieve 100% fluency according to both http://duolingo.wikia.com/wiki/Fluency and https://support.duolingo.com/hc/en-us/articles/205933480-What-does-my-Fluency-Score-mean . The highest possible fluency one can achieve is in the 50-60% range. These recent posts ( https://www.duolingo.com/comment/24890594 and https://www.duolingo.com/comment/24880852 ) also talk about something similar to your question. I hope I helped and if I find anymore resources that could be of importance, I'll let you know!
That's not correct. I currently have a French fluency of 62% http://screenshots.bauffman.be/2017-10-20-20-35-10.png