Could a tax return reporting criminal gains be used as evidence in court?
I suppose it could. I don't believe the IRS coordinates with law enforcement in general. However, if it was a return processed by hand & they suspected something, then I guess they could alert law enforcement.
If you don't report the income, you could get in trouble by the IRS for tax evasion. Just ask Al Capone.
Yes, tax returns can be subpoenaed for a criminal case, as necessary,.