Chiro, on the other hand, advised not cycling over 20-30 miles on a regular basis. Her concern wasn't falling, but posture and its effect on the neck for long periods. My goal is to ride a century or two.

Are her concerns about posture (and overuse, I guess?) legitimate, or is she being overly cautious? I'm determined to find the right bike (upped my bike budget and am leaning toward a Ruby after advice here), I'll pay to have someone fit it correctly if necessary, and I'm not adverse to any sort of stretching/strengthening regimen to make sure that the neck stays safe. But will the prior neck issues pose a problem?
Would it be a big deal for you to be in a more upright position, ie. not have dropped down handlebars? For now, cycle over smooth surfaces and have shock absorbers to reduce vibration.

I know several cyclists who no longer cycle in dropped down handlebars because of neck /posture issues for very long distances/several hrs. per day of riding.