So, he explains Bengali Islam simply as a consequence of shifting Bengal Delta.
Two questions -
1) Any read up on how that Bengal Delta shifted and how more and more forest land were cleared for rice fields?
The line of maximal water flow has always shifted around once the Ganga passed the rocky outcrops of the Rajmahal Hills, the Giridih-Jasidih locations. For instance, to give you a parallel example, the Hooghly was the main stream at one time, and itself broke into three at Triveni (three braids of hair); one stream went east, one went down the middle but not along the exact path that it follows today, and a major stream broke out and joined the sea independently to the east. Of the central course itself, at the Kidderpore Basin, it went out through its main course, the Buri Ganga, now known as Tolly's Canal, while the other, smaller stream continued down south past Diamond Harbour and land's end. Today, the Buri Ganga is a thin, shrivelled up remnant of its old, sea-faring vessel predecessor, while the main channel has also silted up due to the meddling with hydraulic engineering that finally led to the Damodar Valley Corporation.
It was when the stream shifted from the Hooghly to the Meghna-Padma that the change in Bangladesh began. The consensus is that it was due to tectonic shifts and the effects of an earthquake that caused this change. There are thought to be similar reasons for the change in flow of the Teesta, the Trisrota, that flows down from Sikkim and falls into the eastern river complex somewhere in Rajshahi.
Please read that bit above with caution; I always get confused by the Meghna, Padma combination, so don't take the names I use for granted, without checking them on a map.
2) There's a Sanghi trope that eastern Bengal had more Buddhist influence so they were converted easier than west, how much it holds water?
Well, the fully developed line of thought is that Sasanka, a Saivite, was extremely harsh with his Buddhist subjects, that this attitude was interrupted by the Palas, but came back with greater force under the Sena emperors, and that the landlords continued to grind the faces of their tenant farmers into the mud, and that led to a pent-up resentment and anger that made the work of the missionary Sheikhs all the easier. There is no doubt that landlords were harsh with their tenants.
Also, he mentioned that, Islam in south Asia is an rural-agricultural phenomenon which seems true to certain extent like Punjab, Sindh and Bengal but not clear in other cases though one simple observation can be made that there's virtually no converts from Vaishya/Baniya castes (largely urban dwelling castes)- Agarwal, Oswal, Marwari, Khatri (Muslim exists but low population), Bhatia, Arora are all Hindus (or Sikh) but Lohanas have two large Muslim communities - Memons and Khoja Sheikhs. Are there any other trading community you know of?
I presume your question is, do I know of any other trading communities that remained untouched by Islam and therefore bear out his proposition that the traders were less affected than rural and agricultural workers.
Well, yes, I do, with regard to Bengal, and that consists largely of the communities whose caste-names end with -banik - bania. Examples are Suvarna-banik, Gandha-banik, Kansa-banik, and so on, translating to 'gold-smith/ gold-dealer', 'perfumery merchant', 'brass merchant', and the like. I have never earlier thought of them in this context, but do know that they effectively kept the Marwari out of East Bengal, although the community had penetrated West Bengal thoroughly.
