...if a world class footballer comes from Crimea, would he be selected by Russia (RFU)?
Of course, he would. If he would be a Russian citizen and would like to play for RFU-team himself ))
In FIFA eligibility rules it is not sufficient to be a citizen of a country in cases where several associations share the same nationality (this was discussed on the SuppGB thread). Of course, CFU is not in FIFA, but for the purposes of this thought experiment let us suppose it is.
Let us suppose that the CFU is a FIFA member and that Crimea is internationally recognized as part of the Russian Federation, and that both the CFU and RFU are two FIFA associations with shared Russian nationality.
Then a player born in Crimea would only be eligible to play for RFU if he had a parent/grandparent born in some non-Crimean part of the Russian Federation.
Similarly, in the case you mentioned, China would not necessarily be able to select a Hong Kong-born player unless the player had a parent or grandparent born in mainland China. Since FIFA's player eligibility is based on "territory of the association".
What intrigues me about this wording "territory of the association" is whether it is based on *now* or *at the time of birth* ?