My two cents:
Newars are not a caste, rather they are a linguistic community, a conglomeration of Tibetian, Indo-Aryan and Astroloids/dravidians. Look at diverse physiognomies the Newar people flaunt; most of Shresthas, Pradhans, Mallas etc. resemble Chhetris, with a slight hint of chinky eyes. I read somewhere the surnames like Shrestha, Pradhan, Maskey, Rajbhandari, Joshi, Malla, Amatya etc. are called Chha-thare Newars (transliteration of Chhetri?). Even before annexation to Gorkha, Nepal Khaldo had Khas andThakuris, where did they go? Most probably all became Newars.