Adding the car behavior to an object causes that object to interact with solid objects. The only reason you would add the solid behavior to the car is if you want other objects with behaviors that react with solids to interact with it.

You'll just have to apply the effect to every layer you want affected. The performance cost is going to be the same regardless, unless optimizations are made in the engine for this specific case -- e.g. compositing the lower layers and applying the effect, then doing the same with the upper layers. Even in the best case, the effect is being applied twice, which is a lot considering that both applications are full screen.

If possible, you should simplify your layer structure to reduce the number of layers you need to apply the effect to.

Disable the solid behavior on the car when the player gets inside. The car -- seeing itself as a solid -- seems to be colliding with itself.

It's working fine for me when I create a similar setup. There may be something wrong with your events elsewhere.

There are multiple tutorials demonstrating how to do this, and I myself made an example to the same effect.

The "Set Vector Y" action sets the Y velocity of the platform behavior. If, for example, you have jump strength set to 1000 setting vector Y to -1000 would produce the same effect as jumping with that strength.

For the timer, you could just use an instance variable and increment it every tick.

No problem, I hope you get it working. Also, one last thing that I realized: to force a jump, you'll probably have to set the Y velocity of the platform behavior directly -- give it a negative value of magnitude equal to your jump strength -- since simulating the jump control while in the air won't work.

If you're referring to the sprites that are bundled with Construct 2, they're royalty free and can be used in your works without issue.

You could start a timer when the player leaves the ground, not due to a jump. If the player presses the jump key while the timer is below some threshold, trigger a jump. If it's above the threshold, do nothing. Reset the timer when the player hits the ground again.

You can use the System object's Save and Load actions to save and load the entire game state. You could also use the Local Storage to explicitly handle what is or isn't saved/loaded.

There's no way to do that. Just add a blank subevent with the action you want to run after your other subevents.

Pinning is fairly straightforward. Given two objects, A and B, the pinned position of B on A at distance Dist through the angle Angle is found through the following two equations:

B.X = A.X + cos(A.Angle + Angle) * Dist

B.Y = A.Y + sin(A.Angle + Angle) * Dist

Setting Angle equal to 180 would place object B "behind" object A assuming that the image for object A is facing to the right.