Final post for me on this topic.
After weeks of calling and waiting, I called OnStar again today to say that I still could not send addresses to the NAV with either the MyChevrolet or the OnStar App (same thing, at this point, I believe). Neither app displayed the Vehicle Status or Navigation tabs. Both apps correctly indicated that I am on the Basic Plan.
After some runaround, Liz, the rep I spoke with, dug a bit deeper and reset my Basic Plan status and, voila, the app said "updating" and both tabs appeared! Both apps now work and both still say I am on the Basic Plan.
Apparently there is more than one Basic Plan. If your app has lost the ability to send addresses to the in-dash NAV or to see basic status information, call OnStar tech support and tell them of my experience.