(A) The passage discusses one area in which a modern principle has replaced some common-law rules of evidence- the area of admissibility of evidence. It doesn’t mention a wholesale replacement of all of the common-law rules of evidence with modern principles.
(B) Less rigid means more flexible. The inflexibility of eighteenth-century evidence law is indicated with “mechanically excluded” (line 17) and “morass of evidentiary technicalities” (line 21). Modern evidence law is more flexible in its absence of mechanical exclusion and technicalities.
(C) The passage doesn’t tell us that any of the current laws don’t derive at all from common-law doctrines. We don’t even know whether or not Bentham’s nonexclusion principle has some basis in common-law doctrines.
(D) No, maybe it was the beginning of modern evidence law, but that doesn’t mean there was no evidence law before the late 18th century.
(E) No, it doesn’t say that it was only in the 18th century when rules of evidence began to be based on common law.
Hope this helps 🙂