The doctoring need not alter other people's balances, only inflate those of certain "friends".
Have you done any accounting in your life? Obviously not. You cannot inflate the balances of anyone without lowering other balances. And you will have to do this by altering trading records. Bank records and the blockchain are in the hands hands of other people.
As you pointed out yourself recently, there are many accounts with substantial BTC and/or money balances that have not had any activity for years. I cannot imagine who would "forget" hundreds of thousands of euros, unless the owner died or that money was "dirty" in some way. Some of those accounts could have been used to doctor the database -- e.g. by switching the balances of a dead account and that of a friend.
By changing trading records for years back, and have those match the other records all the way? Good luck with that. The fees has to match as well, and match previously leaked databases. The high valued old accounts are known to be locked accounts, btw. Bitcoinica, Baron, etc.
Money deposits and withdrawals can only be verified by obtaining MtGOX's bank accounts, many of them closed long ago. Bitcoin deposits and withdrawals cannot be verified independentlry without knowing the addresses used by MtGOX. It is not obvious that the new management will have access to those records -- all of them -- and they have no obbligation to release them -- all f them -- to the clients.
The criminal investigation will have access to all of them, and their findings should match what other investigators find. Another complicating factor if you want to fake trades.