Lisa, et al..
Would you mind posting the sections and subsections of the fundamental laws which pertain to the succession? I, too, am under the impression that the heir to the throne must have an orthodox mother at the time of birth.
It does state clearly in the fundamental laws that the heir to the throne cannot marry until his wife has adopted the Orthodox faith, but I presume that this applies only to the Tsarevitch, and not to dynasts in general as Maria Pavlovna was allowed to marry Vladimir withour converting, as were several others.
As per who is actually the heir, it is clear that Nicholas II did not want the throne to pass to the Vladimirovitchi. He was, of course, completely familiar with the Laws of the Succession, and in his abdication, he chose his brother Michael (who had contracted a morganatic marriage without the permission of the Emperor -- two strikes against him dynastically) rather than his Cousin Kirill, who, if everyone is to be believed, fit all the dynastic rules.
By ignoring the dynastic succession rules here, it seems that Nicholas basically broke them down in favor of strict primogeniture, which leads us back to... Kirill and Vladimir. If you then ignore morganatic marriages, and only go by primogeniture, you get, as Lisa says, the Illynsky boys.
Interesting question.
nick