One reason why the IRS might not be able to close that audit:
https://www.propublica.org/article/...nal-revenue-service-global-high-wealth-audits
Keep in mind, nobody knows the tax code. Some people have a subject matter expertise, say in collection procedure, but as a whole? It's a mess. Gets worse every year. Actually twice a year.
It could stand a total rewrite. But that will never happen. Too many special interests. Not only big money, but things like child tax credit and the EITC. Can't say when it happened, but the tax code went from revenue creation to social engineering. But that is a rant for another day.