Can blood from dead people be used for blood transfusions and the like?

Noooooooooooooooooo.

Once someone is dying the blood become cold and loses its nutrients and its starts to coagulate so its useless.

When you die, gradually, your cells due. Typically, blood cells are the most fragile, and quickest to die. When your heart is no longer beating and you're no longer beating, the blood is not only stagnant, thus cooler and able to coagulate, it is also lacking the oxygen it needs in order to carry out functions. So no, you cannot do blood transfusions on dead people. However, there are various "shelf lives" on organs. But to answer your question, only live people can donate blood. Also, you should not objectify a dead victim. They are not cars you can take parts from just because the engine is no longer running. Tissue donation is only done when the deceased gave consent before they were dead.

No. When the blood stops circulating around the body, it separates into the different components of the blood: red blood cells, white blood cells, platelets and plasma (...etc). So there will be red blood cells at the lowest part of the body and plasma at the highest parts. Eventually the red blood cells will start to die.

after five or so minutes the red cells explode from lack of oxygen....and the blood is useless

No way!