The schedule is what you see here
Not very fixed...
We have also talked about seing a bit of Napa valley.
How much time should we calculate if we include Pt Reyes, Yosemite and possibly Napa in a north leg of our trip before we´re heading south towards Monterey? I guess we would need to find a place to stay up north, but I´ve understood finding places to stay close to the national park can be a bit difficult/expensive, or do I not need to worry about that?
17 mile drive is on the list yes
Is it worth doing Highway 58 rather than following Highway 1 further down along the coast through San Luis Obispo and Santa Barbara?
Appreciate all the good advice , thanks
Any car museums along the way worth mentioning?
(I´ve seen "The auto collection" at Imperial Palace in Vegas)
Not sure what your schedule looks like. Picking up a car 9/6 with an arrival in Las Vegas of 9/19 gives you almost two weeks. Yosemite would be a bit over 3 hours east of San Francisco. Pt Reyes a little over an hour north. If time allows you should try to fit at least one in. Muir Woods is only 11 north of the Golden Gate. For that matter Sausalito, right across the bridge should be on the list.
Monterey area, Pt Lobos is a nice spot to spend a few hours, would assume you already have 17 mile drive on the list.
Hearst Castle- sounds like your taking Highway 1 down and it's right off of it. Be sure to take some time in Big Sur. Near Hearst Castle- the beach with the Elephant Seals. Cambria is a nice little town if your spending the night.
Backroad drives- Highway 58 might fit for you (it connects Highway 101 to Highway 5). Likewise heading south from San Fran, you can take Highway 35 (Skyline) into the San Jose area, then Highway 17 (or more back-roads) to catch Highway 1. Santa Cruz has a beach and boardwalk that is worth a stop and right across the bay from Monterey.
Santa Barbara in another city worthy of a stop.
Death Valley would be a good hour side trip on your way to Las Vegas and most likely still providing lots of heat (not that Vegas would be cool).