the only things I found is that House Sparrow is separate in two main group domesticus and indicus, bactrianus belong to indicus group. Birds from the indicus should be whiter cheeks. Bactrianus is said to be larger and more pale than others from the same group.
There is also parkini subspecies from western Himalaya that is migrant, but should show a more extensive black on the bib.
There is two things that struck me on the bird from Dayawan, it is the fine black line bordered the upper cheeks and the very pale grey on crown and nape. But I wasn't abble to find anymore about bactrianus description. Size is difficult to judge since it is not given in many places to compare tree and house Sparrow in the same flock, but I would like to say that this bird looks paler than the House Sparrow I have in my garden in France...Agree with John, that the pale bill refer to bird in winter.
About this 3 species often threated as escapee in HK, well one of the advantage of Dayawan is that it is quite far from any big cities, and this is quite a small area, for the moment I still haven't been abble to resight any of this birds a second time. It means these birds are very likely on movment.
I allready give my opinion about Russet Sparrow, I don't know what is the nearest known population, but I wont be surprised there is some unknown isolated population somewhere not very far from here.
About the Eurasian Turtle Dove, well it is a bird known to extent its range, the bird I saw had a typical migrant comportment, I remember have seen it arriving from very far landed on the electric wire for a moment and then flying away quite far again. So I see no reason why this bird could not be of a wild origin, there are not seen elswhere in Shenzhen, and in HongKong the local population still seems to be very restricted to a few area including Mai Po, where I reguraly saw them.
About the House Sparrow, I'm a kind of optimist guy, and the fact that birds has been allready found in two distant different locations at a similar period where there is no previous records, make me believe that they are very likely of wild origin, especially since no one of these birds shows cage dammaged feathers. In the case that some birds would have been release, I see no reason that 2 of them stay in Long Valley and that some others moove 50 kms away...
One thing is sure, is that I will check every Tree Sparrow group this winter, in Guangdong or even in Guangxi to see wether it is isolated sightings or a more general events...
Finally, all of this 3 species has been seen in Autumn, I would probably had a different opinion if would have been in summer for example.
All the best,
Jonathan