^ I think at least some of the threads left dangling will be resolved in later books of The Fall series.
Picard's withholding his knowledge of Cardassian involvement in the assassination is likely to be revealed sometime in the sixty day period leading up to the next presidential election. Given what we've already seen of the new president's relations with Cardassia, I can understand Picard's reluctance to immediately give him this new information. We (and Picard) also know that the Tzenkethi may be involved as well, so it is not clear yet exactly what is going on.
As for Garak, I think it will be fascinating to see what happens next in his journey. In my opinion, flawed characters struggling to redeem themselves are often much more interesting than characters who always make the right choices. I look forward to seeing the next chapter of Garak's story (which I hope will be written by Una McCormack)!