If you do drive in one day then you need to listen to what your guide and driver says and leave when they say to and stop when they say to, the road off the plateau and down from Nyalum to the border is a difficult road especially if you are tired , and that does not include the drive from ebc to the main highway and then from the highway to the edge of the plateau.
eventhough the road has been improved as the above post says , you are still dropping from Nyalum to zhangmu in altitude about 1500 metres, in a couple of hours so this means lots of curves and concentration required., after the last maybe 6 hours of driving from the ebc to nyalum ....
this will mean on the day you arrive at ebc is the day you will have to try and see the base camp probably, ( but could be puke making if you have already come up about 700 metres already to Rhongbuk that day, the base camp is another 200 metres ascent so you will be going well up over the recommended 500 metres ascent in a day to minimise altitlude sickness).. so you can leave very very early the next morning back out, like maybe 6am
I would not push them so you get there to cross the border even if you do it in a day plan to stay in zhangmu and cross the next morning,