When you look at the history of slavery in the US, you have to consider the history of slavery around the world, because it is certainly not unique to the US. Slavery has been around and accepted since the dawn of man. Up until around the 19th century, slavery was pretty much accepted all over the world. Even though it is now illegal everywhere, forms of slavery are still being practiced in places around the world and human trafficking is still a problem.
There was no such term as the "slavery age". Slavery was legal from the beginning until it was made illegal. Africans enslaved other Africans and sold them to European slave traders. Europeans traders brought them west to where manual unskilled labor was most useful. Slavery first appeared in the Western Hemisphere in the Caribbean to harvest sugar cane for sugar and rum. Slavery originated in the US area when it was a BRITISH colony and were imported for labor in cultivating the new world. Slavery was not abolished in the British Empire until 1843. It hung on in some US states because of the need for lots of manual labor on plantations.
To answer your questions:
There may have been a few laws that regulated the treatment of slaves, but for the most part slaves are property and could be treated as the owner sees fit. Some owners treated them like respected servants, while others treated them like animals.
Generally slaves were not paid.
Generally slaves in the US were treated as subhuman, a treatment in some states and by some people that lasted more than 100 years after slavery was abolished in the US. The fact that in the US slaves were a different race made this feeling easy.
Slaves could be freed. Thomas Jefferson freed his slaves upon his death.
Technically, a free man could not be enslaved, so a white man could not be enslaved (but he could be an indebted servant). Slaves originated as imports or children of existing slaves.
I am sure that there have been many cases of murdered slaves, either directly or though poor living conditions. However, since slaves were property, murder or lack of care would destroy the value of that property.
At the time of the Declaration of Independence, slavery was legal by both the US and Britain. The DOI was not a legal document so the statement "All men are created equal" was not law and not directed at slavery (you have to take it in context with the document). There was LOTS of inequalities between at that time as there are now. However, the statement did lead to the abolishment of slavery in Massachusetts.