Yep, you guys have got it right as per my understanding . . . It takes 25 years before you're clear of the EPA and the DOT agencies. However, once you make 25 years, importation is relatively straightforward, though some people get the assistance of a customs broker. I did it myself and it was a tense two hours waiting for them to announce what problem they found to cause me vast amounts of trouble, but they didn't find anything.
Interestingly, though, I found out later that they could have. Turns out that they could impound a vehicle if it had not been steam cleaned, including the bottom, and CamperMog's underside had no end of ex-Sahara sand and gunk. I assume that this is still enforced, perhaps even more vigorously than before. Having CBP impound the vehicle because it is dirty is not the end of the world, but it would cause much inconvenience and some expense.
I'm sad that Margaret's van now has an additional strike against it. It's pretty easy for CBP to know that the vehicle didn't leave when the year was up, and so the VIN will be flagged whenever someone does try for a legal importation. Kind of a messy situation.