Hi, Neil!
I moved this thread over to the California & the West forum. I have not read everyone's posts - just kind of skimmed over a few of them before I moved the thread - so I may be repeating what you've already been told.
I can't speak about the Northern California or Las Vegas portions of the trip, but I can definitely give input about the Southern California segment!
Disneyland is nowhere near Universal Studios (probably about 1 hour apart by car, and that's the minimum - more like one hour, 15 minutes). They are in two totally different counties, with Anaheim/Disney being in Orange County and Universal Studios/Universal City being part of Los Angeles County.
Knott's Berry Farm is also part of Orange County, as are Newport Beach, Huntington Beach, Laguna Beach, etc.
Six Flags/Magic Mountain is way out in Valencia - far away from everything and totally inconvenient!
Knott's (In Buena Park) and Disneyland are relatively close together by car, but more than a few minutes away from each other.
Universal Studios and Hollywood Boulevard are relatively close together by car, but it's still a bit of a drive (maybe 15-20 minutes or so) to get to Universal City.
You could feasibly do L.A./Hollywood and Venice Beach on the same day. You could maybe drive from Hollywood to Santa Monica, perhaps, and see the famous Santa Monica Pier (if that interests you) and then over to Venice Beach.
Some other suggestions:
You could visit the (Original) Farmers Market & The Grove (they are adjacent to each other, at the corner of Third Street and Fairfax) in L.A. - which are both great places to eat, and also places where you can often see celebrities strolling around.
There's also the Los Angeles County Museum of Art (LACMA) and the La Brea Tar Pits - which are adjacent to each other. They are within a few minutes of Farmers Market and The Grove, by car.
Another suggestion is Long Beach - which is kind of in between L.A./Hollywood and Anaheim/Disneyland (but it's part of L.A. County). It's maybe 20 minutes by car from Anaheim, and 25 minutes from Hollywood/Beverly Hills.
In Long Beach you can visit the Queen Mary, the Long Beach Aquarium, and perhaps hop on the Catalina Express for a day trip over to the beautiful Avalon/Catalina Island (which is my other favorite place in Southern California besides Disneyland!) for some buffalo sightings, flying fish tours, dolphin tours - or maybe just a ride in a golf cart for a few hours.
Anyway, good luck with your planning! Have fun!