I'm going to just answer about the difference between the two functions qcut and cut because it's a very important difference:
the first qcut is indeed about quantiles, which means that it's about dividing the data into bins, each containing an equal number of points. For instance if you use deciles it means that there is the same number of people in the ...
This information is in the documentation of Seaborn.
They show a bootstrap confidence interval, computed by resampling units (rows in the 2d array input form). By default, in seaborn version 0.8.1 it uses 95% of confidence interval, which is equivalent to a standard error. This value is a parameter that can be changed.