Hello, I'm glad you enjoyed the article, and thanks for the good questions.

The Graphene will need lose electrons as a result, there would be a limit to the propulsion as a result. An electron capturing device could be utilised, based on a magnetic field. However the purpose of this propulsion is to be used in the final sequence of a manoeuvre. The manoeuvre would be completed before the deplition occurs.

The wavelength of radiation gives each photon it's specific energy, these are absorbed by the Graphene. The ejection process occurs due to "hot electrons", and not like the photoelectric effect. 

A sheet would not be used for the propulsion but a carefully constructed bulk-graphene-sponge. You would want to avoid shining the laser on the edges and more towards the central area, but the geometry is not too important.