[Note: This report and the following one are recaps of answers GRRM gave to various questions during Archon, rather than verbatim quotes of his answers.]
1) Would it be exceptional for a junior member of the Kingsguard to be made Lord Commander over Brothers who have worn the white for a longer time? Or is the Lord Commander position something chosen solely on ability?
The Lord Commander of the Kingsguard is picked by the king. Seniority might play a part, but ability would also be considered.
When was Jon Snow born in relation to the Sack of King's Landing? Around the same time, a month earlier or later?
GRRM stated that he would have to look at his noted to be able to answer but doubted he would answer this question anyway. ;)
What's Hardhome? It's on the map of the area beyond the Wall, but it's never mentioned in the book.
Hardhome is the closest thing that the Wildlings have to a city.
Did Tywin Lannister's campaign against the Reynes and Tarbecks play a part in him being named Hand of the King?
A: It played a small part. Aerys and Tywin knew each other from their youth. Aerys was 19 when he took the throne and wanted a young court, not a bunch of old men around him. Jahaerys was 39 when he died.
Where did the idea of the Red Wedding come from?
A: It is loosely based on the "Black Dinner" from Scottish history. The king invited the sons of his dead rival to a dinner. He feasted them and then had a final dish served to them, with a single drum playing in the background. The dish was a black bulls head, which was the symbol of death. The sons were then executed.