As students of archeology you are all well aware of the importance of information in written form to piecing together the history of a civilization. When we have it, as in the case of the Greeks and Egyptians, it provides a window back through time; a view through the eyes of someone who may actually have been witness to the circumstances and events in question.
1. why is it "the Greeks" , "the Egyptians" instead of "the Greek", "the Egyptians"?
2. why is it "who may have been witness" instead of "who might witness"?
and should'nt it be "witnessed" after "have been"?
In , the word "Greek" functions as a singular count noun. It refers to one person. In  the word "Greek" functions as an adjective. It describes the noun "man". In , the word "Greeks" functions as a plural noun. It refers to more than one person. The same distributional facts hold true for the word "Egyptians".
Modals "may" and "might" are near synonymous in North American English. Below, the main verb is "have been"; i.e., HAVE + -en. "witness" can be replaced with "a witness":
 . . . someone who may/might actually have been (a) witness to the circumstances and events in question.