There is currently no support for handling null/nan values. The series will simply connect the two consecutive valid values, skipping over nans.

The way allowCrossingPaths works is basically as follows: 
By default the range area series will find the min-max for every plotted point and plot all the max values on the high path and the min values on the low path.
If you allow these paths to cross, then the series won't bother taking the min-max values and will simply allow the high path to dip below the low path.

Once the paths cross their fill doesn't really look any different, since it's still the same fill path. The option of doing this was discussed and ultimately considered to be something outside the scope of the range area series at the time. To do this the series would have to find all of its intersection points and use multiple closed paths. 

Here's a pair of range area charts with and without crossing paths.

http://bit.ly/1OkNG6q

https://www.dropbox.com/s/c4cvaw7xww97iff/RangeAreaSeriesCrossing.zip?dl=0