This is not a difficulty, as explained. There are many factors. Your analysis is far too simplistic, but we have come to expect that from skeptical minds. It is better to hear than not to hear, as I have explained. Therefore, your whole attempted dilemma fails utterly.
Missionary work helps people to be saved on an individual basis.
The more people hear the Gospel, it stands to reason that more will accept it and devote themselves to God. It would only be grossly unfair if God demanded that only those who heard can be saved, but since He does not, it is a non-issue, and your concerns are null and void. Preaching the gospel leads to some people getting saved who wouldn't have been if they hadn't heard. That is ridiculous, even if there are millions of other factors affecting salvation. Perhaps that is your confusion, so that you think you see contradictions, where there are none.