What we have done is establish a basic premise. That premise that:
-- Person A's unfaithfulness will prevent zero elect from regeneration. The converse of that is: Person A's faithfulness is not necessary. That is the basis of the accusation that calvinism is not concerned with evangelism, and is the basis of those calvinistic groups that discourage evangelism because it is unnecessary. The problem, of course, is that the calvinists will claim that they have affirmed the "duty" of Christians to proclaim, and that somewhere in the decree of God there might be a rationale for this duty that in which it might make a difference that cannot be defined in this realm.
Regarding Free Will one can say that: Person A's faithfulness might assist one of the "any who will believe" to come to regeneration. This is the basis of the accusation that free will advocates think that they are responsible for someone's salvation. Again, it over-reaches because it does not recognize the word "assist" and replaces it with the idea that the believer is "responsible for."
IOW God foresaw the faithfulness of A in planting the seeds and praying for B and because of the faithfulness of A, he chose B.
A Moderate Calvinist might say that.
I personally don't understand why there would be any question that Calvinists should evangelize. If God commands us to do something, we should do it - it's not disputable. The Bible is filled with God commanding us to spread the Gospel of Jesus Christ.
Whether or not God uses our actions for His purpose should be a non-essential condition for obedience. If God does use us as instruments of His Grace, all the more wonderful.
Is your question concerning God ordering us to do something that may appear pointless? If that is the case, how about loving your enemies? That commandment appears to violate human reason as well. If we truly love God, we will naturally obey His word without complete comprehension of God's ordained will.