Skimmed the thread, some things might have been covered.
Short answer for original question, blunt nose until you hit supersonic speed.
But... at 30mph none of it matters much. You're just not moving through enough air to actually have substantial drag. As dhandes above said, focus on ditching weight and making it work reliably. Whatever you do do not add a spoiler. At 30mph it will give you nothing.
At 30 mph aero has very little effect so the weight of any aero parts would be worse than the gain of the part.
Disagree.
30 mph is fast enough for aerodynamics to play a roles. This includes drag and downforce. What's different between 30 mph and 300 mph is what you actually focus on. At 30 mph, you might just use something "simple" like a blunt nose and sharp tail, and avoid wings and other crazy things, but you still need to engineer the simple shape as much as you would engineer a set of wings.
A spoiler would also have an effect but they are inferior to airfoils. I wouldn't use either though if drag is the biggest concern. Instead use an underbody diffuser if you want downforce. 30 mph is plenty to generate more downforce than the weight of the diffuser.
Dimples decrease the surface area exposed to the airflow. This reduces friction with the air, the friction being what slows the air and increases the boundary layer thickness which causes flow separation.
On the front surface of the ball the reduced surface area takes immediate effect, with the air flowing over air trapped inside the dimples as opposed to flowing over what would be more high-friction surface area. This reduction in surface area causes the reduction in friction buildup which extends the separation point. The benefit of the smaller wake is less parasitic drag.
Dimples increase area. Their purpose it to create turbulence as turbulence resists flow separation more than laminar flow. This means the wake behind the object is less and there is less drag.
The downside of turbulent flow is increased skin friction drag. Dimpling the whole car would be a bad idea. You would want to find the area where separation occurs and then trip the flow at that point. Golf balls have dimples all over because they can fly in any direction.
As far as shaping the body of the car, look at low Reynolds number airfoils. Reynolds number (or Re) can be used to gague how turbulent the air is. Laminar flow occurs between Re = 0 and 500,000 to 1,000,000 (you can trip is earlier than this though), turbulence occurs past 500,000- 1,000,000. You want to keep laminar flow for as long as possible by producing a favorable pressure gradient. The simplest way to do this is constant or increasing curvature. Decreasing curvature starts produces adverse gradients which lead to turbulent flow.
I don't believe skin friction decreases, in fact I believe there's an increase due to a turbulent boundary layer. However, the flip side is, as you said, the turbulent boundary layer allows a better attachment and a smaller low-pressure drag component. The reduction in low-pressure drag simply trumps the slight increase in skin friction.
Exactly.
EDIT
Also, closed wheel is better, as open wheels basically create a vortex that consumes kinetic energy. Don't worry about frontal area too much. It matters, but when you have a streamlined shape, skin friction and induced drag are the major sources of drag. Skin friction drag increases with total surface area.
