How can this be verified
Validators ultimately decide what builders they’re comfortable accepting blocks from. The protocol itself isn’t prescriptive about how they make this choice. In practice you might have some validators that require a full audit for every software update, and others that are comfortable with open source code that’s passed certain tests as an example.
How an on-chain contract would help with safe multiplexing
In the absence of relays you need a way to facilitate counterparty discovery between validators and builders. One idea is to create a registry of eg. TEE builder endpoints on chain. This would create a single accessible point for builders to sign up, and for validators to get info on how to connect to many builders.