How do you escrow goods which cannot be validated?
That Ethereum address containing UTRUST tokens was a good example.
The seller and buyer cannot trust the escrow with both the private key and the public address. The escrow cannot check whether the private key unlocks the public address.
Do not complicate things Try to read the whitepaper and you will be enlightened.
"Enlighten me then"

But seriously read white paper and you will know everything that you should know
Alice sells the private key of her Ethereum wallet to Bob. Bob pays through UTRUST. Alice is notified that the money is in escrow and sends the private key to Bob. Bob claims the key does not work. Alice opens a dispute.
Tell me how the UTRUST Mediator resolves this transaction.
Good question I haven't thought so much about. But you are right, a situation like this can hardly be solved probably. But maybe if Alice gives the private key to the mediator? Something someone would probably never would like to do...
But there are probably other situations where stuff could happen. If someone claims that a good has not arrived, when you can't track the package. When I sold some stuff on ebay a lot of people wanted to go for the cheapest sending option, which is not trackable. So if something happens you can't check for example.