Concerning the Kingsguard, and the fate of Margaery Tyrell
At the end of A Feast for Crows, Margaery Tyrell was the prisoner of the the Faith (as a result of Cersei's spectacularly incompetent political machinations, which also resulted in Cersei suffering the same fate. Daft cow. Cersei's evil plan relied upon the fact that a member of the royal family, when selecting a champion for a trial by combat, must (by tradition & law) select a member of the Kingsguard to defend him/her. Thus, Margaery's champion to defend her from accusations of fornication with Osney Kettleblack must be selected from the following:
Jaime Lannister (who only has one hand and, besides, is Cersei's twin brother and unlikely to willingly risk his life to defend the honour of Cersei's arch-rival)
Loras Tyrell (as Margaery's younger brother and the finest knight in the Seven Kingdoms, he'd be the obvious choice- however, Cersei realised this and manipulated him into leading the attack on Dragonstone. He's now at death's door, and in no shape to defend anyone's honour).)
Osmund Kettleblack (as the brother of the person Margaery's fornication has ensnared, and a pawn of Cersei, he's also unlikely to risk his life to defend, etc)
Balon Swann (a decent chap and a good fighter, but currently on his way to Dorne to give Gregor Clegane's head to the Prince, hence unavailable)
Boros Blount (Fat, old and unlikely to stand a chance in a combat)
Meryn Trant (see above)
Arys Oakheart (in Dorne defending Princess Myrcella, and thus unavailable, but see below)
So it seems Margaery would be forced to call upon Blount or Trant to defend her honour, who wouldn't stand a chance against whichever champion the Faith selected to prove her innocence. This was precisely as Cersei planned; however, she didn't count on being accused of fornication herself, and being in the exact same situation as Margaery. Cersei's plan was, of course, to call her brother to fight on her behalf (the idea being that then they'd both die together. Not the stablest of thinkers, our Cersei).
HOWEVER
The one piece of information that's missing here concerns Arys Oakheart. Near the end of A Feast for Crows, Oakheart was killed by Areo Hotah when the crazy-plan-to-crown-Myrcella went wrong. Nobody in King's Landing knows about this yet, but Balon Swann is on his way to Sunspear to deliver Clegane's head- as soon as he gets there, there'll be a raven headed for King's Landing bearing news of the death of a Sworn Brother (and probably mentioning Myrcella's horrific facial scar, which should bear some interesting fruit). Which means that a new member of the Kingsguard will need to be selected. Who gets to select new members of the Kingsguard? Why, their Lord Commander, of course- Jaime Lannister, Cersei's twin brother and erstwhile lover, who has just been urged to ride to her rescue.
My guess is that Jaime will select Garlan Tyrell (Margaery's other brother, who hasn't had a huge amount to do thus far in the series other than showing himself to be a thoroughly nice bloke at Joffrey's wedding, and having been repeatedly remarked upon as being one of the finest swordsmen in Weseros) as the new knight of the Kingsguard- he'll then fight to champion Cersei (who Jaime will want to save because, well, he's her brother) and Margaery (in an attempt to salvage the Lannister-Tyrell alliance and appease Mace Tyrell, last seen marching on King's Landing with a huge fuck-off army). This way, Cersei gets to live long enough to see her remaining children dead under golden shrouds before being strangled by her little brother (as foreseen by Maggy the Frog- incidentally, my bet is that the "little brother" is not Tyrion, as Cersei seems to think, but Jaime).
What the newly-armed Faith with have to say about all this is, of course, another story.
Concerning Jeyne Westerling, take two
When we're treated to Cersei's flashbacks to Maggy the Frog telling her future, she describes Maggy as the wife of a spice merchant (elevated to the petty nobility by Tytos Lannister), who brought her back from the East. Cersei met Maggy in Lannisport, so it would be reasonable to assume that Maggy lives there. Also, when Cersei tells the story to Taena Merryweather, she identifies Maggy as a maegi- a wise woman/shamaness.
When Jaime meets Jeyne Westerling, we find that her mother was a Spicer- from a family of Lannisport merchants raised to the nobility by Tytos Lannister. The mother's mother (Jeyne's grandmother), was apparently a maegi from the East, who was brought to Westeros by her Spicer husband. Does this sound familiar?
So it appears that Jeyne Westerling's grandmother is Maggy the Frog. Whether this has anything to do with anything, or is just a bit of backgroundy stuff, is anyone's guess.