Bob can send a voucher to a third party acting as escrow. Escrow checks it, creates a new voucher, and gives it to Alice.
I was thinking more of a trustless escrow, such as Bisq's 2-of-2 multi-sig escrow between buyer and seller, rather than just shifting the trust from one party to another. I can't see a way to do this in the current system.
It should do. The whole point of the blinding is that even ChipMixer would not know which certificate is which. They would be unable to log any activity cause all they can tell is if a certificate is valid and unspent, but not when it was signed or who has owned it.