Stalk hardening begins as floral initiation occurs and may not be a good indicator of the best time to harvest. Delaying toppiong till full bloom allows the stalk to harden before topping. This would eliminate stalk hardness as a useful tool for determining maturity under these circumstances. A tobacco leaf that breaks easily when bent signifies maturity and is a relatively easy and nondestructive test. According to this book, Japanese beetles are not listed as a pest of tobacco.
Aphids are and carry at least two diseases.
However, apparently it is impossible to prevent these diseases by killing the aphids because they are so numerous and the viruses infect the plant minutes after the aphid latches onto the plant. The aphids do secrete a sticky substance called honeydew that can drop onto leaves. BUT it allows the growth of a black fungus which can deteriorate the quality of the tobacco it grows on. I personally have aphids and I spray with liquid Seven to try and keep the population down. The planting of tobacco at two foot intervals in rows 3 feet apart is derived from studies of differing plant spacing.