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!
In French- 100 % is impossible. It's good if you are at 40, very good if you are at 50, brilliant if you are at 60, but 100 is impossible.
That's not correct. I currently have a French fluency of 62% http://screenshots.bauffman.be/2017-10-20-20-35-10.png
Some courses low fifties. With courses learning English you can get into the seventies.
I'm 68% but now the questions seem to be very easy. Easier than some months ago. Weird?
I imagine that's partly due to the "practise effect." As you get more fluent, it's probably harder and harder for the algorithms to challenge you as much as when you first started out. I envy your skill level. ;-)