That's what financial reports are for, which he has given. His personal income tax returns would not be that complicated.
With all the leaks, hints, etc. about the investigation, this is the first time in a long time that his taxes were mentioned. I find that very strange if there was anything to talk about. I guess we will see though.
No, tax returns have Fed tax ID numbers. ....and, if you file a fraudulent return, its a crime. So, they are essential for forensic accounting and essential for charging, should fraud be found. It is very unlikely that many, if any, of the Trump entities have audited statements. Internally prepared statements are helpful to understand the magnitude funds flow, but otherwise do little in this type of investigation. Financial statements (audited or not), though, would lack the detail (such as Taxpayer ID numbers) that you need to trace ownerships.
Real estate developers do almost every thing via pass through entities (partnerships, S-Corps, LLC's), so the results of the various operations tend flow to the personal return. This would make his personal tax returns would be very complicated. That said, his holdings could aggregate at the Trump Organization, though that is likely an LLC....and, the tax returns of the Trump Organization as a wholly-owned, private company, would also be in the hands of Mueller.
The complications here are that each real estate holding would likely be comprised of several pass-throughs, each owned with a unique ownership structure. There would likely be numerous outside partners, many of them silent. It will take awhile to really figure out who owns what....and who the real partners are once you trace around the numerous shell companies.
I do hold a CPA, so you may have to trust me on this if you do not know.
But, the financial and ownership complexity of real estate is what makes it ideal for money laundering, which is pervasive in NY real estate.
How foreign investors launder their money in New York real estate
There is a very good chance that Trump is tied up in international money laundering....and, could well be the thing that makes him behave like he is guilty of something. That said, I admit my statement is only informed speculation. But, I believe it to be true.
BTW .... there have been no leaks from the Mueller camp. You don't know exactly what they are up to. It makes you wonder if the Donald is getting much sleep.