So, the divided loyalty argument is not so much pertaining to thoughts in the mind of a President, as it is a matter of law between nations. The Founders sought to avoid such entanglements and intrigues, and legally structured eligibility to office in the executive branch in such a way as to preclude them legally, which was the only means at their disposal.
I agree. The founders particularly wanted to keep foreign influence on the Commander-in-Chief to a minimum, because of what was going on in Poland in the 1772. The partitions of Poland were enabled in no small part to foreign influence exerted on the Polish government by calculating foreign powers who often bribed Polish officials to exercise their veto in the legislature. The situation effectively rendered Poland a vassal state of foreign powers and it was partitioned up over the 1770s and 90s. In the US, the Electoral College was also devised to prevent foreign influence on the election of a President which would inevitably occur if the choice of President was given to the legislature. Here is an interesting article on the Natural born Citizen Clause and its history at the Constitutional Convention.
http://faculty.maxwell.syr.edu/jyinger/citizenship/history.htm