Indeed a lot of questions .
I think the best way to find out is to google inside the JPL website for position detemination and orbit correction .
IIRC the deepspace network determines the position , together with some advanced doppler measurements of the signals NH produces . The camera onboard also registers the pictures NH takes and compares to the stars charts .... A lot of redundant measurements in order to increase the accurancy .
The actual position of NH may be retrieved in the Neodys Minor Body website . Thats what I did some time ago , in order to find out the trajectory of NH .
http://www.orbitsimulator.com/cgi-bi...num=1241108229
It seems NH will only need a few minor burns to arrive right on place and shedule ...!