Its culture is totally Bangladeshi culture. - Actually other way round. All of the Bengali greats become great because of cultural influence of Kolkata. Language your biggest identity you speak diff dialects which you might not understand , but we formalized the write the way you write.
Kolkata controlled the faith of Bangladesh for more than 250 years..
Islam - Let say WB become 100% Muslim, do you think the Muslims will like to unite with Bangladesh? As you know the border is open, but never economic migration happens from WB to Bangladesh but other way round. All rickshawalas and all fruitsellers are Bangladeshis. Just saying.
Mamta - You are ... why providing false information. Her ancestors has nothing to do with Bangladesh. Her family originated from Rarh region in Bengal.
Assam - Assamese and Boros just hate you.. period. Its a BJP ruled state now... Even pre poll predictions are telling BJP will have more foothold in Bengal.
One advice, i believe you now must be understanding Hindi as well. Just a friendly advice, see Mungerilal ka Hasin Sapne in youtube.