Listen to Billy Connolly or Craig Ferguson if you want to know what a real Scottish accent sounds like. Whatever accent Jimmy Doohan was doing, it sure as hell wasn't Scottish.
Jimmy Doohan actually talked about that quite frequently. It was his take on what a Scotsman might sound like 300 years in the future. He discussed it at some length with Tom Snyder on the Tomorrow Show in a 1976 appearance that also featured Koenig and De Kelley