I think what you are missing is that validating servers/clients would not allow non-validating servers to rejoin the cluster (i.e.: servers/clients with validation enabled will validate both outgoing and incoming connections).
As I see it, by the time you restart the leader (and hence quorum switches to the non-validating portion of the cluster) all of your clients will suddenly fail (they are still validating, and there's no good server for them to connect to). Conversely, if you restarted the clients first they would all become unavailable before the quorum switch happened.
As I see it, by the time you restart the leader (and hence quorum switches to the non-validating portion of the cluster) all of your clients will suddenly fail (they are still validating, and there's no good server for them to connect to). Conversely, if you restarted the clients first they would all become unavailable before the quorum switch happened.