Mandarmani to Kolkata distance is about 180 kms. The best option is to book a cab and enjoy the scenery along the way. One way cab booking gives you the best route experience. The ideal route would be via NH 16 and NH116B. This is the fastest route and takes about 4 hours despite normal traffic.
The best time to visit Kolkata is right after the monsoons, as autumn(October-November) sets in the city sets in the spirit of Durga Puja. Larger than life pandals, beautiful durga idols, various delectable dishes. If you want to soak in the actual spirit of Bengal there can be no better time. The place is a bit crowded though during the Durga Puja. The winters(December to February) in Kolkata are pleasant. This is perhaps the best time to go sightseeing in Kolkata. The Kolkata book fair is also held around this time. The summer(March to May) season is hot and humid and best avoided. The Monsoons(June to September) do bring some welcome relief from the soreing sun. However, the daily life is highly disrupted as streets are flooded during monsoon.
Outstation cabs are easily available from Mandarmani. You can book a cab and head towards Kolkata via NH 16 or the NH116B route. However these two routes are longer compared to the usual preferred cab ride.
The roads are a in excellent conditions and there is nothing to worry. Just book a cab in Mandarmani and enjoy your trip.