Dante was taught Thomas Aquinas's proposition that good and evil are more than the mere construct of humanity's opinion and consciousness. Rousseau believed that reason (rational faculties) could not be absolutely trusted and that good and evil were constructs of human passions. Therefore, things Rousseau would be guilty of in Dante's eyes and that he might condemn were discounting the power of the reasoning mind and excusing good and evil as the natural result of humanity's passions.
We need to remember that these two characters are very different in terms of their perspective and beliefs about God and sin. As the masterful response in #2 points out, in some ways they could be characterised as being diammetrically opposed, especially in the way that Rousseau freely flaunts his sin with a specific desire to shock and not as a real "confession." In answering this question, therefore, we need to remember the very different position of both of these two figures and their points of conflict.
I think that Dante would have a problem with Rousseau's lack of belief in God. There is little in Rousseau that echoes Dante's fundamental notion of a redemptive force that can provide salvation. Where Dante would place Rousseau because of this is all over the map. I don't think that there is a realm of the inferno into which Dante would not place Rousseau because of his lack of belief in God. Rousseau makes little mention of God in the confessions, for the work is all about him and nothing to do with the power of the almighty. For this, I cannot see Dante not placing Rousseau in several level of the inferno for his lack of belief in God. In terms of specific sins, I think that this, too, becomes an area where Rousseau is placed in many regions of the inferno because Rousseau "confesses" so freely what he has done and who he is, but does so without any real hope or desire for external salvation. If anything, Rousseau uses his own sins and transgressions, according to Dante, for public consumption, which in its own right earns him many places in the inferno. This might be seen in the last circle of the inferno, as Rousseau is willing to receive praise or "salvation" from a society as opposed to God, indicating a severe betrayal of the almighty. For example, Rousseau freely admits to his lustful desire for older women, but does so in a manner that is meant to shock or even challenge the sensibilities of the social order and not for any real notion of spiritual salvation in Dante's mind.