1. Bengalee Muslims are largely of Afghan origin except in the South and East where they are of Arab descent. Bengal-Arakan coast was one of the earliest to receive The Message conveyed to them by Sahabi Abu Waqqas. The Sahabi was the maternal uncle of the Holy Prophet and was tasked with this mission even before the Hizrat. The target was the growing Arab / semi-Arab community located in this region as a result of the east-west, ie, Canton-Aden trading since the Monsoon winds was discovered by the Byzantine navigator Hippolatus.
2. The Sahabi had stayed among our midst for 8 years travelling between the ports of Swandip and Akyab-Ramree and Sylhet. That is why the present day Arabs accept the Rohingyas as their people. Moreover, certain Zionist groups claim Mizos are a lost Israelite tribe.
3. There is a huge misconception about conversions from Hinduism in SA. There were no forced or coerced conversion whatsoever. Otherwise SA would have been totally green after a thousand years of Muslim presence and rule. What really ocurred is that surplus Brahmins (those not involved as priests) and Ksyatrias had found it convenient to convert to the religion of the rulers for entry into the army and govt service. These Hindus, however, did not abandon their clan/tribal identity. That is why throughout SA you can find Thakurs, Mazumdars, Gujjars, Dogras, Jats, Rajputs, Chowdhurys, Biswas, etc among Muslims as well as Hindus. Bhuttos are a Rajput clan. So is Zardaris. Raja/Rao is Rajput.
4. The Low Cast did not convert into Islam at all. The reason is simple. Their vocation was better pursued remaining a Low Cast / Dalit. The Muslim rulers also needed the menial workers for their empire. The Low Cast were targeted aggressively by the Christian missionaries allowing these people to continue in their professions as sweepers, barbers, cobblers, etc.
5. Whereas Arab/semi Arabs were growing in numbers in the E and S, in 1202 Qutubuddin Aibak's Turkish General Bakhtiar Khilji formally conquered Bengal overthrowing the Senas who had become quite unpopular with the Buddhist population.