The problem with arguments based on scriptures between two people who both believe in the scriptures, but read them differently, is that it will be difficult for either to be convincing. This is a common problem in many modern discussions of the meanings of scriptures. Different people read them differently, and sometimes, even without a faith tradition, we have brothers or sisters who might read them differently.
Sherem perhaps understood that he would not be able to confound Jacob through words, so he attempted to have Jacob fail at what might be seen as a reasonable request. Sherem suggests that, if Jacob believes that this is the message of the old prophets, and that he is claiming place among them, that he should simply show a sign, and then all would believe.
Jacob removes himself from the suggestion of power and declares that Jehovah himself will provide that sign. It would be that coming Messiah himself, who would show a sign upon the person of Sherem, the unbeliever. Jehovah does something to Sherem whereby Sherem becomes so weak that he cannot even feed himself. Others are required to feed him for several days.