Hi Maciek
Yes, now a days Rocket goes from Morolgonj due low water level. But you can go Bagerhat, visit the 60 domed mosque then head to Hularhat to catch the rocket (The Rocket is available that day). In worse case, if there is no Rocket, the the Launch (Modern 3 story boat) to Dhaka from Hularhat.
I think if you are not unlucky enough, you will reach Kolkata within 12 p.m if you take Saturday night bus. I will recommend to take bus of Shohag Paribahan.
But if you dont want to take risk, you can take reverse trip. say :
27 Feb : Take direct bus to Dhaka and reach at night (say 8 p.m). Take rest.
28 Feb : Whole days Dhaka trip. Take Rocket steamer to Hularhat. In that case u have to book tkt earlier)
Mar 1 : Reach Hularhat. Take bus to Bagerhat. Viist 60 domed mosque. Take bus to Khulna. :
March 2 : Viist some places in Khulna. at 3 p.m : take bus to Benapole. Stay in Parjatan Hotel at Benapole.
March 3 : Cross the border in early morning (7 am) and take Kolkata jeep bus to reach Kolkata around 11 am.
Note : I suggested this because due to delay in Ferry (Fog/Low water) sometime, bus delays 5/8 hours to cross the river. If so, you will fall in trouble. But if you take my suggestion, you can avoid that risk.
BTW, million of thaks for your kind words toward me.
Mahmud