The Skull does not burst during cremation.
Instead the skull becomes fragile from the heat and eventually crumbles.
There are a number of natural openings which would allow pressure build up to be relieved.
The largest of these is the "foramen magnum" where the spine connects.
In life, these openings are sealed with soft tissue, sort of like a gasket would seal a mechanical joint.
But the brain is not a fully sealed vessel that would contain any significant pressure.
The body does not sit up during cremation but due to the high heat of the cremation fire the body can move some and change posture.
This position is characterized as a defensive posture and has been seen to occur in bodies that have experienced extreme heat and burning.
The pugilistic stance is "caused by high-temperatures in fire, resulting in muscle stiffening and shortening and can occur even if the person was dead before the fire.
The body does not feel pain during cremation.
When someone dies the person does not feel any pain or have any senses anymore as the brain has completely shut down and every organ has also shut down.
So the person being cremated will not know they are being put into the cremation oven and being cremated.
If the person feels pain then they are alive and not dead.
But if the person is indeed really dead then the person will not feel or experience any pain or discomfort from being cremated.
Cremation although it seems crude is actually a better option than burying a body and it's more environmentally friendly as well.
The body is burned up and the bones are ground up into powder which takes less space than a body being in the ground.
Also no coffin is needed and no concrete for burial vaults are needed either.
Cremation can cost thousands of dollars less than traditional burial of the body.