We already have this type of escrow service represented by bisq, so if you do not provide enough features, you will not find anyone using your service, especially as it requires trust.
There is no difference between 2/3 multisig address or full trust in the third party because they can scam you at any time. Perhaps the damage will be less in the first case.



How will you combine fees, decentralization and multisig addresses? Suppose that a B party sends the money to a multiple address and the two parties (A "reciver" & B "sender" ) agree to release the escrow, how will you receive your money (C)?