pak-one as someone of rohilla background I have to inform you that there has been a lot of intermixing in areas stretching from rampur to saharanpur, it was inevitable after centuries of coexistence. The issue there is that with the intermixing, some of the caste has crept in but you are right, nowhere close to what we see elsewhere in the whole zaat and what nots.
Sure, intermixing will definitely occur. Rohillas, as you know, are still patrilineal therefore the authenticity/lineage can not be doubted. It becomes highly suspect for other Muslims in India who claim “Pathan” ancestry. Maybe they have some linkages, maybe they don’t.
Also I read an interesting article on Pakhtuns in the West Indies/South America who had been transported during the British colonial period and have still retained much of the culture despite more extensive intermixing with other Muslim groups.