The Great Hall

So I just re-read the first book, and I began to wonder what lead up to the decision to move the stone to hogwarts in the first place... And I have some questions... Lets play follow the stone...

We know that Quirrell, with the help of Voldemort who was at the time possessing Quirrell, made an attempt to steal the stone from Gringotts. This attempt failed due to Hagrid taking the stone out of its vault before Quirrell could get to it, on Dumbledore order to do so.

1. What prompted Dumbledore to think of moving the stone? What piece of evidence or intelligence did he receive to make him think Gringotts was no longer a safe place?

2. How did Dumbledore know someone was after the stone in the first place? What events took place to tell both Dumbledore and Flamel that the stone was no longer safe in France? Was there an attempt at stealing the stone earlier? Why did Flamel make the choice to move it to Gringotts? Was there an early indication that it was Voldemort, or was that only revealed once Harry faced him at the end of the book?

3. Did the Mirror of Erised contain the Stone the first time Harry encountered the mirror? Or was there yet another prompting for Dumbledore to enhance the stone's protection? Was Dumbledore simply on his way to retrieve the mirror, for the purpose of hiding the stone within it, when he ran into Harry using the mirror?

We know the Guardians had already been set up by Halloween as evidence of the troll and Fluffy. But if the Mirror was not down there at the time, as proof by Harry finding the mirror after Christmas, where was the stone, really the first half of the book? Dumbledore's office? Somewhere else safe?