'This could actually work.', was Cana's evaluation of the plan Mark proposed.
But the feasibility of the plan was never the deciding factor here, the most important deciding factor was trust.
And Cana was, in a single word, a very distrustful person.
She already had her hands full trusting her leader, not knowing what went through the boy's mind. And now, she had to trust a person she just met?
That was impossible for Cana, and that was precisely the reason she chose to agree with the plan.
To 'prove' herself, to 'change' herself. 'Things that were impossible for me once, I will do them now.', she had resolved herself.
Of course, having a load of expensive artefacts that made escape possible in almost any situation played a big factor in her agreement.