Do you really believe that Smart Contracts can replace a lawyer?
It seems that you have to go back to doing research, what is called a smart contract and what is its main function.
You cannot connect between smart contracts with lawyers, they are two things that are mutually contradictory. The function of lawyers is to provide services to make legal agreements based on state letters. While smart contracts are the rules for running the system.
And has anyone actually tried to have any kind of exchange and used one Smart contract?
Some exchanges that use smart contracts are those from the ICO. so that investors believe that the project runs according to the rules of the smart contract. You might be able to read some ICO based exchanges. for example, bancor protocol.