How does vasoconstriction/vasodilation change blood pressure?
The circulatory system is a dynamic system which cannot be adequately explained by your example (at least not by me). You need to understand it, not seek to make it fit your (especially) electrical circuits with resistors. Blood isn't electricity. At least try a fluid dynamics model.

Let's take this very simple model: Blow up a balloon four-fifths of the way, and put a wide inflatable cuff around its middle. Inflate the cuff so that the balloon bulges just a tiny bit at the ends, then stuff it all into a plexiglass box so that the balloon has no room to expand. The plexiglass box represents our body. Lets call the pressure inside the balloon now normal blood pressure. The air in the balloon represents your total blood volume. It can't change from moment to moment; it's fixed. The box can't change it's volume moment to moment either. It's fixed.

The cuff represents arterial smooth muscle. Vasoconstriction can be represented by inflating the cuff further. Constriction (inflation of the cuff) will increase the pressure throughout inside the balloon, because the same amount of gas now has to exist in a smaller space. Vasodilation (deflating the cuff) decreases the pressure inside the balloon, because the gas can now inhabit a larger amount of space.

That's it, really. If the same volume must inhabit a smaller, constricted space, the pressure exerted by blood in that space will be higher. If the blood vessles dilate, the pressure in the blood vessels falls.

Now add about 20 layers of complexity to that simple model, and you have a working model of the circulatory system.

Sparknote answers:


Arterial and arteriolar smooth muscle "injects energy into the system", resulting in systemic increases in the pressure existing in the vascular "circuit" (meaning somewhat elastic tubing) if resistance requires energy. (So I have misunderstood: see @Raoul's answer.)
Sorry, I didn't read this properly the first time around. Yes, the heart supplies the energy. The contribution of the elastic walls of the arteries is not active, but passive.
Absolutely. The more elastic/compliant the arteries are, the less work the heart must exert to pump the blood through the circuits. The stiffer and narrower the arteries are, the harder the heart must work to pump blood through the circuits. The result of that increased work is a thickening of the muscular walls of the heart, called ventricular hypertrophy, which is a sign of elevated pressure in the system.
I'm not sure I want to commit here to your lingo, but the answer should be inferable from numbers 1-3. Elastic vessels help (by rebound compression) to propel the blood through the circuits. Stiff vessels hinder it, making the heart do more of the work.


I am not an engineer, so I may be misusing some of the terms. 

Edited to add: Please see @Raoul's answer for a better explanation.