I hope Van isn't Tedd's brother, or that if he is this fact isn't revealed until after Magic has made it's decision. Tedd needs to be on her A game right now; the last thing she needs is the emotional trauma of learning that the mother she feels abandoned her went on to have another child who she actually stayed with.
I really hope these three aren't the only eligible seers. In addition to the points you made, all of them are Caucasian or part-Caucasian, and unless Dan throws us a major curve-ball none of them are full-time females. If these three are supposed to represent the world, they're seriously lacking in diversity.
Yes, but that would only change her for up to a maximum of one month. She couldn't even make a change to male be permanent, the way a change to female is made permanent by pregnancy.
Tedd wouldn't want to be one sex permanently, anyway. I doubt she'd even want to stay in male form for the full month; she's always liked to spend a good amount of her free time in female form.
Assuming spells from the old magic system still work in the new system. I still think the old spells will at least need to be modified in order to be used in the new system, if they can be made to work at all.
I figured Diane and Susan still counted since the started as "maybe sisters" and seem to have ended as "sisters in spirit".
And the storyline involving Magus, Sirleck, Elliot, and Ellen is still building upon the consequences of the first Sister story, so I think that alone would be enough to justify the Sister III title.