What officially published adventures contain yuan-ti

The books are designed to let the DM choose how to connect them. There is no 'you must play XYZ book before ZYX book' ordering, except insofar as levels restrict. (It would make no sense to finish Rise of Tiamat then jump into Curse of Strahd, for example, since COS ends at a lower level).

While SKT is written with the assumption that HotDQ/RoT has already occurred, there are even notes inside it on how to invert that paradigm if desired.

Because they are the more "pure", i.e. most human

According to the AD&D 1st edition Monster Manual 2 (1983), yuan-ti are humans who were corrupted into snakelike creatures by some ritual or other:

Inhabitants of tropical jungles, the yuan ti are a degenerate and corrupt race of creatures who were once human. All are devout demon worshippers who have a high regard for all kinds of reptiles. Through dark and unknown practices, their blood has become fouled and they produce monstrosities. There are 3 types of yuan ti: Purebloods, halfbreeds, and abominations.

While the etymology is not strictly defined in this entry, it is clear that the "purebloods" are most human, while the snake-like traits are referred to as a form of corruption, or fouling of their blood. The most obvious answer is that the purebloods are so named because they are the most humanlike and therefore least "corrupt".

However, it is the "corruption" that raises one's status within yuan-ti society, since they value the snake-like traits more than the humanlike traits. In other words, being pure-blooded is a bad thing to the yuan-ti.