Like a minor, a Gentile is halakhically incapable of steering an halakhic process of change. Similarly, the gemara in Sanhedrin 57b maintains that a Gentile who commits adultery with a married woman who has already engaged in bi'a has committed 'gilui arayot' and can be punished according to halakha. This gemara confirms that only through bi'a can a Gentile construct a marriage accepted by halakha. Obviously, these cases demonstrate the unique nature of bi'a - bi'a is not your standkinyan. In truth, each of these halakhot is based upon the aforementioned principle.
Symbolic acts are merely devices or tools which are used to establish a halakhic change.
Effectively, the author behind these acts and his counterpart are orchestrating the change themselves, and only employing the ma'aseh to trigger the change. The author retains the right to delegate the symbolic act to a shaliach. I may appoint another to perform the symbolic act of kinyan and still dictate that the ownership be acquired by myself. In such cases I may also hinge the change upon an external stipulation. Even though the act was performed in full, I may, as its author, state certain contingencies which must occur for the change to be valid.