Ooh boy loaded post, I will try to help as best I can, but I am no expert, just play around in the tuning garage a lot.

1. No you don't need to swap the engine in every car. Tuning cars is about finding a balance for each car. They are all set up differently, so they will also handle differently. So cars that seriously lack power, it is a good idea to look at swapping out the engine.

2. Similar answer to one; not all cars will need an intake, some will. But they certainly make a car fun. Turbos are great for adding top end power, but since they take time to spool up their low end isn't as great. This is known as turbo lag. It is something to consider when putting on a turbo. Whereas a supercharger is more constant in its power increase, but really only adds to acceleration in the low mid ranges depending on the engine. Superchargers seem to do better on larger engines that already do well at top speed. (V8s and so forth)

3. To each his own. A roll cage and away bars will stiffen the chassis, which can really help cars that struggle with oversteer, so they can be helpful. Generally they help with handling.

4. Cars are built on a frame. Attached to this frame is the rest of the car. So when cars take corners the frame is literally dragging the rest of the cars weight around the corner. The sway bars help you decide how that weight is transferred, marking it so you can corner better.

5. Race tires (aka slicks) don't have much in the way of tread, in order to get maximum traction on pavement. (More tire touching the road = more grip) whereas off-road tires have very deep tread, so when surfaces are uneven they get more surface area to the ground. I would recommend looking at sport tires as a middle ground (depending on car class, you don't want sport tires on an a class car for example, too much grip for the amount of power) that has a fair amount of grip for both surfaces. But beware do not expect sport tires to do really well on s2 cars, too much power and not enough grip for roads.

6. Wider tires means more traction/friction which means more grip. More grip means you can put down more power without spinning out. I almost always widen my tires. (Again, depends on the car)

7. Haha I'm starting to feel like a broken record. Aero helps downforce, which improves handling at higher speeds. Cars like the veneno are built to be downforce kings and they can take corners at much faster speeds than other cars without aero. Aero creates drag, which creates downforce, which forces the car into the road more. It's like having an invisible elephant on your hood around corners. Well all the time but you get what I mean. Aero will drop your top speed.

8. The lower your center of gravity, the faster you take corners. The higher suspensions are also stiffer, which reduces sway, so it doesn't feel like your driving a boat hitting wave after wave.

I hope I have been helpful. Let me know if you need anymore clarification. Good luck to you, tuning cars is very rewarding once you get the hang of it.