Did Dumbledore and Snape have a plan, for when Quirrell stole the Philosopher's Stone?

Question

In Deathly Hallows we learn that Dumbledore asked Snape to keep an eye on Quirrell. We presume because he thought Quirrell was fishy.

Keep an eye on Quirrell, won't you?

I assume at some point after that exchange after that point in Philosopher's Stone, Snape was talking to Quirrell about the protection of the stone in the Forbidden Forest, presumably conversing on how to get past all the protection.... (Snape might have traded info with Quirrell.)

"... d-don't know why you wanted t-t-to meet here of all p-places, Severus..."

"Oh, I thought we'd keep this private," said Snape, his voice icy. "Students aren't supposed to know about the Sorcerer's Stone, after all."

Harry leaned forward. Quirrell was mumbling something. Snape interrupted him.

"Have you found out how to get past that beast of Hagrid's yet?"

"B-b-but Severus, I --"

"You don't want me as your enemy, Quirrell," said Snape, taking a step toward him.

　"I-I don't know what you

"You know perfectly well what I mean."

An owl hooted loudly, and Harry nearly fell out of the tree. He steadied himself in time to hear Snape say, "-- your little bit of hocus-pocus. I'm waiting."

"B-but I d-d-don't --"

"Very well," Snape cut in. "We'll have another little chat soon, when you've had time to think things over and decided where your loyalties lie."

So obviously Snape (playing double agent as always) and therefore Dumbledore knew it was just a matter of time before Quirrell attempted to steal the stone, did Snape and Dumbledore have a backup plan, as I doubt they planned that the golden Trio was going to save the day.

Answer 1

The plan was presumably originally that the Stone's primary defences (the 'obstacle course') would be too strong to break through. Apparently, though, like many a reader, they decided around Christmas that these were not strong enough after all, and Dumbledore had the Mirror of Erised brought in.

The plan was that the Mirror of Erised would be impossible to overcome:

'How did I get the Stone out of the Mirror?'

'Ah, now, I'm glad you asked me that. It was one of my more brilliant ideas, and between you and me, that's saying something. You see, only one who wanted to find the Stone - find it, but not use it - would be able to get it, otherwise they'd just see themselves making gold or drinking Elixir of Life. My brain surprises even me sometimes ...'

Harry Potter and the Philosopher's Stone - p.217 - Bloomsbury - Chapter 17, The Man with Two Faces

And, indeed, when Quirrell looks at the mirror what he sees is what he really wants - himself presenting Voldemort with the Stone and winning all the glory from his master.

'I see the Stone ... I'm presenting it to my master ... but where is it?'

Harry Potter and the Philosopher's Stone - p.210 - Bloomsbury - Chapter 17, The Man with Two Faces

For a bit more, see this question.