Under the law, private companies are audited for creditworthiness every 5 years.
A group of banks contracts an audit firm (in TFG's case, Moody's). They all provide the information revealed in loan applications, the auditor consolidates them, then does an independent review of asset value.
In 2013, Moody's assessed that the assets were overvalued by $1.7 billion & loans totaled $1.2 billion more than advertised. This brought the net worth from the fraudulent $4.5 billion reported by Forbes to about $1.6 billion.
In 2018, the loan accumulation was found to be $300 million higher BEFORE the Deutschbank Germany, Bank of China, and Saudi loans were identified.
Hence, it appears he was never a billionaire. We still don't know how much he owes Russia, but his idiot kid admitted to borrowing from them.
As to the $400 million inherited, you're dead on. However, we have to remember the no repayment term "loans" (really a tax avoidance scheme) from daddy in 1971-1978. Those "loans" totaled $54 million in 1978.
If it lived off of 100x the median household income for those 8 years, and still had $60 million.
In the 13 years up to the inheritance, that $60 million would have increased to $125 million, while living lime a king & doing nothing.
By 2016, that would have been worth $1.2 billion, and the inheritance would have gone up to nearly $3 billion.
No "business" deals, no branding, no bankruptcies, no stupid TV show...
He would have been worth 5-8 times actual, for doing nothing.
He's up there with the worst businessmen ever.