Neither the 50 or 75 mile route goes over Diablo.
Let's see (I think I've got last year's route map at home, so this is only what I remember, and my memory is not that good...). 50 and 75 mile routes follow the same course, which leaves Pleasanton, up the Dublin Grade to Castro Valley, up Redwood Rd, over Pinehurst and down to Moraga. The 75 mile route then takes Moraga Way out to Orinda. Last year the course was changed, but I continued stubbornly on the old 75 mile route, which takes the Three Bears along Briones Resevoir, then up Pig Farm, then over Reliez Valley Rd and back to Walnut Creek, where it meets back up with the 50 milers. I don't remember how the 50 milers get from Moraga to Walnut Creek. After Walnut Creek it's back along Danville Blvd and smaller streets all the way to Pleasanton. This is the crowded section of the course -- lots of cyclists, cars, etc.
The 75 mile is a fairly strenuous ride, with about 5000 feet of climbing. Pig Farm and Reliez Valley are the two hardest climbs. It's very scenic, well, until the last 20 miles when you're heading back. I love riding the Three Bears, which last year the organizers cut down to One Bear (but I, and many others, ignored the map and signs). I've never done the Wildflower 100, so can't compare the two.
So, you're forming a team?