Sherem is successful in part of his mission. Following the logic of the scenario being built for Sherem's presence among the Nephites, his first mission is to create a groundswell of support. He is clearly an accomplished speaker, and "did lead away many hearts."
It is somewhat appropriate at this point to examine the reasons that Sherem can lead away many hearts. First, of course, is the basis of acceptance he creates with his flattering words. He gains the confidence of his audience by speaking to their pride. Secondly, he preaches based on the brass plate texts. This not only continues the flattery, but also continues to create a basis of commonality. Sherem will not ask them to make wholesale changes in their beliefs, but rather only subtle ones. It is also possible that he saves his most dramatic confrontation with the idea of Christ for his public discourse with Jacob. The last point, not to be overlooked, is that Sherem is probably an excellent speaker.
The modern world can easily underestimate the power of a good public speaker because we have so many means of entertainment that we forget that one of the attractions of a good speaker was entertainment. In the ancient world, there is little available for entertainment and a good speaker becomes a draw for the community. As good speaker, Sherem's audience is predisposed to like him an agree with him, because he is entertaining, and skillful at presenting his arguments. In many ways, the smooth con man is the direct spiritual descendant of Sherem and his ilk.
Jacob does not appear to be aware of all of the contexts swirling around Sherem, but he does understand that Sherem is particularly interested in a public debate with Jacob. Once again the question is, why?
One scenario is that Sherem is so proud of his debating skills that he wishes to test them against Jacob. If Sherem's sole reason for preaching among the people of Nephi is to gain converts, he is safest to deal wit the masses, and avoid the man who would have the most influence against him. He would convert those he could and be happy. He does not do this, however, he earnestly seeks out his potentially most dangerous foe. It is either tremendous hubris, or something else.
It is something else, and a very insidious something else. Sherem has a goal of discrediting the teaching of Christ, but even more directly, discrediting the man who taught about Christ, Jacob. Sherem seeks out Jacob precisely because Jacob is the real target. Under the scenario suggested, Jacob continues to be a threat to the traders even without an official position. Only discrediting him directly frees the traders to act without Jacob's continuing condemnation. Therefore, Sherem seeks out Jacob, and seeks a public debate with him. Only before an audience will the discrediting be effective.