That blog was posted on Nov 9, 2009. This thread is started exactly a month later. There might be some connection. Neither the profile of  "Rajeswor Shrestha" nor the information he provided are trustworthy. It is similar to your reference to "one Nepali Historian", but you don't mention who that "one" is. So, stop such worthless rumors.

As far as the history of Shrestha, the writer of that blog does not seem aware about the position of Syasyah in Newar community. It is not hard to explore the root of Syasyah as they have tradition of Dyu Puja that they have preserved well. None of the Dyu Puja have link outside valley.

The "Pa-ra-dhan" description of "Pradhan" is just a hollow assumption. "Pradhan" do not have any relation to "Dhan". What do you say if someone writes "Adhikari" came from the word "kar"- as they used to be car-mechanics.

All the Shresthas living out of KTM valley had well preserved their language and culture until two generation ago. I have traveled myself to various newar settlements in Ilam, Dhankuta, Baglung, Bandipur, Pokhara, Bharatpur, Tistung, Hetauda etc. where almost all of the older generation Shresthas were speaking Newar. They have their own dialects though. It was Panchayet, which imposed one-language policy strictly and imposed only one language to all new generation people through panchayeti education and banned other languages in education and public use. This cleansing effort of Panchayet has made hard to newer generation Nepalese (whose native language is different from Khas language and were spread in small clusters around the country) to keep up with their native language. 

I have met many Shresthas who hardly can speak Newar language but I have not met a single Shrestha whose grandparents were unable to speak Newar language.