I don't know if I've shared this before on the forum, but my day job is an aerodynamic analyst/designer. I work on planes more than cars, but if there is anything in the book that seems like it doesn't make sense or isn't covered, I could probably help decipher it.
Some comments:
1. Absolute value of Cd is useful when comparing different vehicles, but I agree with deltas being the most useful when designing or modifying something. As long as your modeling/testing is equivalent, changes in Cd are accurate and reliable, and that's all that matters. You know how your car performs at whatever its current Cd is, so whether it's going to improve or not is what you need to know, not the true value of Cd. We're kind of now getting to the point where we'll fly UAVs for smaller programs based solely off CFD, but anything big or that carries people gets multiple wind tunnel tests during development.
2. Steady(well-behaved) flows are pretty easily modeled very accurately in CFD, but unsteady, messy flow is tough. You get into needing dynamic simulations which are very expensive computationally. It's trivial to run yaw simulations, so I'm not sure I agree that yaw is something that will surprise you if you're doing things on a computer or in a tunnel rather than just track testing.
5. Rear wings will always increase drag. I actually worked on a case where an SAE car gained a bunch of downforce with a front wing at zero drag penalty due to the upwash from the wing keeping a ton of air from hitting the front tires, but that's a niche situation. Air dams, flat floors, and diffusers are essentially "free" performance. You can lose both drag and lift if done properly.
6. I don't know if I agree with calling airdams a crutch, but an airdam without a flat floor is leaving a lot on the table. The less air you let under the car, the easier it is to manage and the easier it will be to make your diffuser effective. That's true with or without a flat floor. If you put a flat floor on but leave your front end the same, you're still going to generate a bunch of lift on the bottom of your front bumper for anything shaped like a Z. Modern cars are pretty good at not having much upward-facing area, but old cars with the diving board bumpers really need airdams.
7. Yep, adding aero to just one end of a balanced car is an easy recipe for an unbalanced car. Adding just a mild wing to a car prone to oversteer or a small splitter to one prone to understeer would be pretty safe, but you always have to be aware of the whole system.