All C350e configurations qualify for the government's full plug-in car grant of £5,000 at present.
Once the 50,000 plug-in car grants of the current scheme have been exhausted, a tiered approach based on 'tailpipe CO2' and 'zero emission mileage' will be adopted, with cars placed in three categories. No details have yet been released on the subsidy level that each category will attract, if any.
I believe, though I do not know, that the nine month restriction originates from Mercedes-Benz UK, who are "guesstimating" on the expiration of the current plug-in car grant scheme and their ability to deliver customer cars within that time.