While it may not seem like a lot, you show a .2 or .3 voltage drop to the VR for the sensing line.

What did you use for ground when you tested that? If you used the battery, check it again with the case as the ground and see if it changes.

For now I might run a line to the ballast directly to the positive on the battery and start it. See if the voltage still ramps up to 18 volts.
If so, then it probably is the alt. since you eliminated everything that could affect the vr.
Well there is one other item, the plug itself on the vr, have been known to get loose. But since you aren't driving, I am not sure if that would be an issue.

make sure you run it on the feed side of the ballast.