Fed must hold collateral equal in value to notes in circulation
Section 16 of the Federal Reserve Act, 1913. 
No, the Fed’s accounting generally follows the standard convention, where assets=liabilities+equity. All of its assets count as collateral; no particular assets are “earmarked” as collateral. By construction, the Fed holds assets against all its liabilities (mostly deposits), not just Federal Reserve Notes. 
Yes, you can see the assets and liabilities of the Fed in its H.4.1 release.