I went into ~/Library/Preferences (yet again) and still could not "delete" the help-related plist items. When I opened the files, the title bar indicated that they were locked. However, I could erase the contents manually. ?!?!
I could also (as suggested by X Lab) drag the file to the Trash and also empty the Trash.
Results:
Offline — Help seems to work fairly well.
Online (for info for which "Content not available" pops up upon search when offline) — Hit or miss, about 50/50, but at least 50% of the time it works now.
So, for the moment at least, problem solved and approach noted (ie, Trash, rather than Delete).
