I'm not that puzzled regarding Hessalam.

Moridin said outright that there aren't enough of them, and we've often wondered about this. Most of them are dead.

Moridin needs two qualities in his Chosen for the Final Battle -
1. They must be able to command the Dark Legions effectively
2. They must have individual power.

The reason he created M'Hael is obvious - he's proven himself to be a competent leader and he's also deadly as an individual. That will allow him to command the Shadow against Rand and fight if he needs to. This is also the reason Hessalam is back - one can't get rid of someone with Graendal's skills and power so easily. I think it's idiotic to put them in some old woman's body considering she may need to run around the battlefield as needed, but that's Shadow folly for you.

As you mentioned above - another Chosen may be on the way. We should keep out eyes out for a person with potential leadership abilities who is also powerful as an individual channeler. They won't raise Alviarin no matter how well she does. I also think we shouldn't discount someone being turned to the Shadow against their will and being raised to Chosen. The wagons filled with Fades and Aes Sedai are an ominous pointer and I wouldn't discount a nasty shock like Someryn, Talaan or even Alivia, though the last conflicts with prophecies etc.

Then again, we might be looking at the return of Aginor or Sammael. It would make sense to bring them back, but I find this even less likely.