If Sarah had been eliminated, could Cirie then use the advantage in future Tribals? Or was this literally "I'm giving this to you see if I can catch you trying to betray me, it has no value to you whatsoever."?

I just feel like Cirie could have left the Two vote advantage home, and on the way to Tribal said to Sarah, "hey, we're voting Tai, vote with us if you don't want to risk being eliminated." A little dickish, but not as dickish as a flat out, "I'm stealing the advantage you gave me and using it against you," betrayal.

We do see Sarah whispering to Tai during TC. It's possible she told him that they were going to get Michaela, but she was going to steal his vote to throw Cirie off the scent and he believed it.

Thank you so much for explaining this to me. I had no idea what Cirie was thinking…