[Kollel Iyun Hadaf wrote in Insights to Sanhedrin 79a:]
>>the Rabanan are referring to a case in which there were nine Jews and only >one Nochri. Since the Nochri is "Kavu'a" ("in his place" and not separated >from the others in the group), he gives the crowd a status of half-Jews and >half-Nochrim because of the principle of "Kol Kavu'a k'Mechetzeh Al<<
I would say that Kavu'a is the opposite, that he is separated from others in the group by virtue of his separate status as a nochri. If he is not separated then there is more logic to consider him batel berov. No?
"Kavu'a" means that the object, or the person, is in his place, and is not moving away and separating physically from the rest of the objects, or persons, in the group. It is not defined by his status as being "different" than the others in the group, but by being physically moved away from the group. (In fact, the whole concept of "Kol Kavu'a k'Mechetzeh Al Mechetzeh Dami" applies *only* when one object has a different status than the other objects in the group -- such as Tereifah meat among Kosher meat).
Regarding your question that there is more reason to consider him Batel b'Rov, that is exactly what the principle of "Kol Kavu'a..." is overriding, as the verse quoted in the Gemara here (as a Gezeiras ha'Kasuv) is teaching.
The Gezeiras ha'Kasuv is teaching (as Rav Gustman zt'l explained) that when an object is not together with the rest of the group, we ask, "which of the ten objects that were in the group is this -- one of the nine Kosher ones or the non-Kosher one." Since there are nine kosher answers of "Kosher" and one of "non-Kosher," we follow Rov. When, however, the object in doubt is still among the other objects in the group, we do not ask "which of the ten objects is this," because we cannot single out that object from the rest of the group in our question. We must ask a question that applies equally to the entire group. We are therefore told to ask "is this object Kosher or non-Kosher," a question which can be asked and answered equally for any and every member of the group. When that is the question, the options for the answer are only two: "Kosher" or "non-Kosher." We therefore give a 50% chance that it is Kosher, and 50% chance that it is not Kosher, and consider it a Safek.
M. Kornfeld, Y. Shaw